Stabilize WebGL book pagination restore

public/js/book-pagination-module.js

@@ -19,11 +19,16 @@ class BookPaginationModule extends BaseModule {
         this.latestBlockId = 0;
         this.latestRenderedBlockId = 0;
         this.appliedPageReserveBlocks = new Set();
+        this.preparedBlockCache = new Map();
 
         this.bindMethods([
             'initialize',
             'refreshFromHistory',
             'preparePendingBlock',
+            'getPreparedBlockCacheKey',
+            'rememberPreparedBlock',
+            'takePreparedBlock',
+            'clearPreparedBlocks',
             'buildSpreads',
             'buildPages',
             'buildSpreadsFromPages',
@@ -86,6 +91,10 @@ class BookPaginationModule extends BaseModule {
                 this.setCurrentSpread(this.currentSpreadIndex + direction);
             }
         });
+        this.pages = this.buildPages([]);
+        this.spreads = this.buildSpreadsFromPages(this.pages);
+        this.currentSpreadIndex = 0;
+        this.publish({ reason: 'initial-title-spread', allowFutureUnrendered: true });
         this.reportProgress(100, 'Book pagination ready');
         return true;
     }
@@ -93,11 +102,13 @@ class BookPaginationModule extends BaseModule {
     handlePageCountChanged(event) {
         this.pageFormat?.setPageCount?.(event.detail?.pageCount);
         this.metrics = this.pageFormat.getTextureMetrics(this.pageFormat.getTextureWidth?.());
+        this.clearPreparedBlocks();
         this.refreshFromHistory();
     }
 
     async refreshFromHistory(event = null) {
         const token = ++this.refreshToken;
+        this.clearPreparedBlocks();
         const detail = event?.detail || {};
         const gameId = detail.gameId || this.storyHistory?.currentGameId || null;
         const latestRenderedBlockId = Math.max(
@@ -117,7 +128,7 @@ class BookPaginationModule extends BaseModule {
             this.latestRenderedBlockId = 0;
             this.currentSpreadIndex = 0;
             this.appliedPageReserveBlocks.clear();
-            this.publish();
+            this.publish({ reason: 'empty-history' });
             return;
         }
 
@@ -135,7 +146,7 @@ class BookPaginationModule extends BaseModule {
             : renderedSpreadIndex >= 0
                 ? renderedSpreadIndex
                 : Math.max(0, Math.min(this.currentSpreadIndex, Math.max(0, this.spreads.length - 1)));
-        this.publish();
+        this.publish({ reason: 'history-refresh', allowFutureUnrendered: true });
     }
 
     getContinuationBlockId(latestBlockId = 0, latestRenderedBlockId = 0) {
@@ -158,6 +169,31 @@ class BookPaginationModule extends BaseModule {
         const historyEndBlockId = options.includeUnrenderedHistory
             ? Math.max(0, pendingBlockId - 1)
             : latestRenderedBlockId;
+        const cacheKey = this.getPreparedBlockCacheKey(gameId, pendingBlockId, historyEndBlockId, latestRenderedBlockId, options);
+        const cached = options.activate !== false ? this.takePreparedBlock(cacheKey) : null;
+        if (cached) {
+            this.latestBlockId = pendingBlockId;
+            this.latestRenderedBlockId = latestRenderedBlockId;
+            this.pages = cached.pages;
+            this.spreads = cached.spreads;
+            this.currentSpreadIndex = cached.targetSpread
+                ? cached.targetSpread.index
+                : Math.max(0, Math.min(this.currentSpreadIndex, Math.max(0, this.spreads.length - 1)));
+            if (options.publish !== false) this.publish({ reason: 'prepared-cache-activate' });
+            document.dispatchEvent(new CustomEvent('book-pagination:block-prepared', {
+                detail: {
+                    blockId: pendingBlockId,
+                    spread: cached.targetSpread || this.getCurrentSpread(),
+                    spreadIndex: cached.targetSpread?.index ?? this.currentSpreadIndex,
+                    latestBlockId: pendingBlockId,
+                    latestRenderedBlockId,
+                    preloadOnly: false,
+                    reusedPreparedPagination: true
+                }
+            }));
+            return cached.targetSpread || this.getCurrentSpread();
+        }
+
         const historyBlocks = historyEndBlockId > 0
             ? await this.storyHistory.getBlocksRange(gameId, 1, historyEndBlockId)
             : [];
@@ -181,6 +217,13 @@ class BookPaginationModule extends BaseModule {
             const lines = Array.isArray(spread?.[side]) ? spread[side] : [];
             return lines.some(line => Number(line?.blockId || 0) === pendingBlockId);
         }));
+        if (options.activate === false) {
+            this.rememberPreparedBlock(cacheKey, {
+                pages: preparedPages,
+                spreads: preparedSpreads,
+                targetSpread: targetSpread || null
+            });
+        }
         if (options.activate !== false) {
             this.latestBlockId = pendingBlockId;
             this.latestRenderedBlockId = latestRenderedBlockId;
@@ -189,7 +232,7 @@ class BookPaginationModule extends BaseModule {
             this.currentSpreadIndex = Math.max(0, Math.min(this.spreads.length - 1, this.currentSpreadIndex));
             if (targetSpread) this.currentSpreadIndex = targetSpread.index;
         }
-        if (options.publish !== false) this.publish();
+        if (options.publish !== false) this.publish({ reason: options.activate === false ? 'prepare-preload' : 'prepare-activate' });
         document.dispatchEvent(new CustomEvent('book-pagination:block-prepared', {
             detail: {
                 blockId: pendingBlockId,
@@ -203,6 +246,39 @@ class BookPaginationModule extends BaseModule {
         return targetSpread || (options.activate === false ? preparedSpreads[0] : this.getCurrentSpread());
     }
 
+    getPreparedBlockCacheKey(gameId, blockId, historyEndBlockId, latestRenderedBlockId, options = {}) {
+        const includeUnrendered = options.includeUnrenderedHistory === true ? 'unrendered' : 'rendered';
+        return [
+            gameId || 'game',
+            Math.max(0, Number(blockId || 0)),
+            Math.max(0, Number(historyEndBlockId || 0)),
+            Math.max(0, Number(latestRenderedBlockId || 0)),
+            includeUnrendered,
+            this.metrics?.width || 0,
+            this.metrics?.height || 0
+        ].join(':');
+    }
+
+    rememberPreparedBlock(key, prepared) {
+        if (!key || !prepared?.pages || !prepared?.spreads) return;
+        this.preparedBlockCache.set(key, prepared);
+        while (this.preparedBlockCache.size > 12) {
+            const oldestKey = this.preparedBlockCache.keys().next().value;
+            this.preparedBlockCache.delete(oldestKey);
+        }
+    }
+
+    takePreparedBlock(key) {
+        if (!key || !this.preparedBlockCache.has(key)) return null;
+        const prepared = this.preparedBlockCache.get(key);
+        this.preparedBlockCache.delete(key);
+        return prepared;
+    }
+
+    clearPreparedBlocks() {
+        this.preparedBlockCache.clear();
+    }
+
     buildSpreads(blocks = []) {
         this.pages = this.buildPages(blocks);
         return this.buildSpreadsFromPages(this.pages);
@@ -891,11 +967,11 @@ class BookPaginationModule extends BaseModule {
 
     setCurrentSpread(index = 0) {
         this.currentSpreadIndex = Math.max(0, Math.min(Math.round(Number(index || 0)), Math.max(0, this.spreads.length - 1)));
-        this.publish();
+        this.publish({ reason: 'set-current-spread', allowFutureUnrendered: true });
         return this.currentSpreadIndex;
     }
 
-    publish() {
+    publish(options = {}) {
         const writtenPageLimit = Math.max(0, (Math.max(0, this.spreads.length - 1) * 2) - 1);
         document.dispatchEvent(new CustomEvent('book-pagination:spread-updated', {
             detail: {
@@ -904,7 +980,9 @@ class BookPaginationModule extends BaseModule {
                 spreadCount: this.spreads.length,
                 writtenPageLimit,
                 latestBlockId: this.latestBlockId,
-                latestRenderedBlockId: this.latestRenderedBlockId
+                latestRenderedBlockId: this.latestRenderedBlockId,
+                reason: options.reason || 'publish',
+                allowFutureUnrendered: options.allowFutureUnrendered === true
             }
         }));
     }

public/js/book-texture-renderer-module.js

@@ -117,14 +117,25 @@ class BookTextureRendererModule extends BaseModule {
         this.addEventListener(document, 'webgl-book:page-count-changed', this.handlePageCountChanged);
         this.addEventListener(document, 'book-pagination:spread-updated', (event) => {
             const spread = event.detail?.spread || this.pagination?.getCurrentSpread?.();
+            const spreadIndex = Math.max(0, Number(event.detail?.spreadIndex ?? spread?.index ?? 0));
             const latestBlockId = event.detail?.latestBlockId;
             const latestRenderedBlockId = Math.max(0, Number(event.detail?.latestRenderedBlockId || 0));
             this.currentSpread = spread || { left: [], right: [] };
             if (latestBlockId && Number(latestBlockId) > latestRenderedBlockId) {
                 this.markPendingReveal(latestBlockId);
                 const id = String(latestBlockId);
+                if (event.detail?.allowFutureUnrendered === true && !this.activeAnimations.has(id)) {
+                    this.drawSpread(this.stripUnrenderedLines(this.currentSpread, latestRenderedBlockId), ['left', 'right']);
+                    return;
+                }
                 if (this.activeAnimations.has(id)) {
                     this.revealPublishBlockIds = new Set([id]);
+                    const visibleSpread = Math.max(0, Number(window.BookLabDebug?.getBookState?.().spreadIndex || 0));
+                    const flipActive = document.documentElement.dataset.webglPageFlipActive === 'true';
+                    if (!flipActive && event.detail?.allowFutureUnrendered !== true && spreadIndex > visibleSpread) {
+                        this.drawSpread(this.currentSpread, ['left', 'right'], { preloadOnly: true });
+                        return;
+                    }
                     this.drawSpread(this.currentSpread, ['left', 'right']);
                 }
                 return;
@@ -143,10 +154,24 @@ class BookTextureRendererModule extends BaseModule {
         });
         this.addEventListener(document, 'story:manual-scroll', this.fastForwardAnimations);
         this.addEventListener(document, 'story:history-restoring', this.stopAnimations);
+        this.currentSpread = this.pagination?.getCurrentSpread?.() || { index: 0, left: [], right: [], pageMeta: { left: null, right: null } };
+        this.drawSpread(this.currentSpread);
         this.reportProgress(100, 'Book texture renderer ready');
         return true;
     }
 
+    stripUnrenderedLines(spread = {}, latestRenderedBlockId = 0) {
+        const latestRendered = Math.max(0, Number(latestRenderedBlockId || 0));
+        return {
+            ...spread,
+            left: (Array.isArray(spread.left) ? spread.left : [])
+                .filter(line => Math.max(0, Number(line?.blockId || 0)) <= latestRendered),
+            right: (Array.isArray(spread.right) ? spread.right : [])
+                .filter(line => Math.max(0, Number(line?.blockId || 0)) <= latestRendered),
+            pageMeta: spread.pageMeta || { left: null, right: null }
+        };
+    }
+
     markPipelineTiming(name, detail = {}) {
         const entry = {
             name,
@@ -602,7 +627,8 @@ class BookTextureRendererModule extends BaseModule {
             if (!animation || animation.completed) return;
             regions.push(...this.assignRevealTiming(blockRegions, animation));
         });
-        const sideRegions = regions.filter(region => region.side === side);
+        const currentSpreadIndex = Math.max(0, Number(this.currentSpread?.index ?? this.pagination?.currentSpreadIndex ?? 0));
+        const sideRegions = regions.filter(region => region.side === side && Math.max(0, Number(region.spreadIndex || 0)) === currentSpreadIndex);
         if (!sideRegions.length) return null;
         const bounds = sideRegions.reduce((box, region) => ({
             x: Math.min(box.x, region.pixelRect.x),
@@ -636,26 +662,30 @@ class BookTextureRendererModule extends BaseModule {
 
     collectRevealRegionCandidates() {
         const candidates = [];
+        const sourceSpreads = Array.isArray(this.pagination?.spreads) && this.pagination.spreads.length
+            ? this.pagination.spreads
+            : [this.currentSpread || { index: 0, left: [], right: [] }];
+        sourceSpreads.forEach((spread) => {
             ['left', 'right'].forEach((side) => {
-            const spreadLines = Array.isArray(this.currentSpread?.[side]) ? this.currentSpread[side] : [];
+                const spreadLines = Array.isArray(spread?.[side]) ? spread[side] : [];
                 spreadLines.forEach((lineRecord) => {
-                const region = this.createRevealRegionForLine(side, lineRecord);
+                    const region = this.createRevealRegionForLine(side, lineRecord, spread?.index);
                     if (region) candidates.push(region);
                 });
             });
+        });
         return candidates;
     }
 
     assignRevealTiming(blockRegions = [], animation = {}) {
-        const wordTimings = Array.isArray(animation.wordTimings) ? animation.wordTimings : [];
         const totalDuration = Math.max(
             Number(animation.totalDuration || 0),
-            ...wordTimings.map(timing => Number(timing.delay || 0) + Number(timing.duration || 0))
+            ...((Array.isArray(animation.wordTimings) ? animation.wordTimings : []).map(timing => Number(timing.delay || 0) + Number(timing.duration || 0)))
         );
         const sortedRegions = [...blockRegions].sort((a, b) => {
-            const aStart = Math.max(0, Number(a.wordStart || 0));
-            const bStart = Math.max(0, Number(b.wordStart || 0));
-            if (aStart !== bStart) return aStart - bStart;
+            const aSpread = Math.max(0, Number(a.spreadIndex || 0));
+            const bSpread = Math.max(0, Number(b.spreadIndex || 0));
+            if (aSpread !== bSpread) return aSpread - bSpread;
             const aLine = Math.max(0, Number(a.lineIndex || 0));
             const bLine = Math.max(0, Number(b.lineIndex || 0));
             return aLine - bLine;
@@ -667,25 +697,14 @@ class BookTextureRendererModule extends BaseModule {
         const totalArea = textRegions.reduce((sum, region) => sum + Math.max(1, region.area), 0);
 
         textRegions.forEach((region) => {
-            const wordStart = Math.max(0, Number(region.wordStart || 0));
-            const wordEnd = Math.max(wordStart + 1, Number(region.wordEnd || wordStart + 1));
-            const firstTiming = wordTimings[wordStart] || null;
-            const lastTiming = wordTimings[Math.min(wordTimings.length - 1, wordEnd - 1)] || firstTiming;
-            let delay = firstTiming ? Math.max(0, Number(firstTiming.delay || 0)) : fallbackDelay;
-            let duration = lastTiming
-                ? Math.max(1, (Number(lastTiming.delay || 0) + Number(lastTiming.duration || 0)) - delay)
-                : 0;
-            if (!Number.isFinite(duration) || duration <= 0) {
-                duration = totalArea > 0
+            const duration = totalArea > 0
                 ? Math.max(1, totalDuration * (Math.max(1, region.area) / totalArea))
                 : Math.max(1, totalDuration / Math.max(1, textRegions.length));
-                delay = fallbackDelay;
-            }
             timedRegions.push({
                 ...region,
-                timing: { delay, duration }
+                timing: { delay: fallbackDelay, duration }
             });
-            fallbackDelay = Math.max(fallbackDelay, delay + duration);
+            fallbackDelay += duration;
         });
 
         fixedRegions.forEach((region) => {
@@ -707,7 +726,7 @@ class BookTextureRendererModule extends BaseModule {
         });
     }
 
-    createRevealRegionForLine(side, lineRecord = {}) {
+    createRevealRegionForLine(side, lineRecord = {}, spreadIndex = null) {
         const blockId = String(lineRecord?.blockId ?? '');
         if (!blockId || !this.revealPublishBlockIds.has(blockId)) return null;
         const animation = this.activeAnimations.get(blockId);
@@ -719,16 +738,14 @@ class BookTextureRendererModule extends BaseModule {
             const y = content.y + Number(rect.y || 0);
             const width = Math.max(1, Number(rect.width || content.width));
             const height = Math.max(1, Number(rect.height || this.metrics.typographyLineHeightPx));
-            return this.normalizeRevealRegion(side, blockId, lineRecord, x, y, width, height, this.getImageRevealDurationMs(lineRecord));
+            return this.normalizeRevealRegion(side, blockId, lineRecord, x, y, width, height, this.getImageRevealDurationMs(lineRecord), spreadIndex);
         }
         const rect = this.getLineInkRect(side, lineRecord);
         if (!rect) return null;
-        const wordStart = Math.max(0, Number(lineRecord.blockWordStart || 0));
-        const wordCount = Math.max(1, this.getLineWordCount(lineRecord.line || {}));
-        return this.normalizeRevealRegion(side, blockId, lineRecord, rect.x, rect.y, rect.width, rect.height, 0, wordStart, wordStart + wordCount);
+        return this.normalizeRevealRegion(side, blockId, lineRecord, rect.x, rect.y, rect.width, rect.height, 0, spreadIndex);
     }
 
-    normalizeRevealRegion(side, blockId, lineRecord, x, y, width, height, fixedDurationMs = 0, wordStart = 0, wordEnd = 0) {
+    normalizeRevealRegion(side, blockId, lineRecord, x, y, width, height, fixedDurationMs = 0, spreadIndex = null) {
         const padding = Math.max(2, Number(lineRecord.fontPx || 18) * 0.12);
         const left = Math.max(0, x - padding);
         const top = Math.max(0, y - padding);
@@ -738,11 +755,10 @@ class BookTextureRendererModule extends BaseModule {
         const rectHeight = Math.max(1, bottom - top);
         return {
             side,
+            spreadIndex: Math.max(0, Number((spreadIndex ?? Math.floor(Number(lineRecord.pageIndex || 0) / 2)) || 0)),
             blockId,
             lineIndex: Number(lineRecord.lineIndex ?? lineRecord.pageLine ?? 0),
             fixedDurationMs,
-            wordStart,
-            wordEnd,
             area: rectWidth * rectHeight,
             pixelRect: { x: left, y: top, right, bottom },
             rect: {

public/js/ui-display-handler-module.js

@@ -1074,14 +1074,31 @@ class UIDisplayHandlerModule extends BaseModule {
                 publish: false,
                 includeUnrenderedHistory: true
             });
+            const previewRevealDetail = {
+                id: sentence.id,
+                blockId: sentence.blockId,
+                wordTimings: sentence.animation?.wordTimings || [],
+                cueTimings: sentence.animation?.cueTimings || [],
+                totalDuration: sentence.animation?.totalDuration || 0,
+                spread: previewSpread,
+                preloadOnly: true
+            };
+            if (previewSpread && typeof bookTextureRenderer.prepareRevealBlock === 'function') {
+                bookTextureRenderer.prepareRevealBlock(previewRevealDetail, { preloadOnly: true });
+            }
             if (Number(previewSpread?.index || 0) > currentSpreadIndex) {
-                await this.waitForWebGLPageFlip({
+                const flipped = await this.waitForWebGLPageFlip({
                     direction: 1,
                     reason: 'pending-block-overflow',
                     targetSpread: previewSpread.index
                 });
+                if (!flipped) {
+                    throw new Error(`WebGL book page flip did not start for prepared spread ${previewSpread.index}`);
                 }
-            preparedSpread = await bookPagination.preparePendingBlock(sentence);
+            }
+            preparedSpread = await bookPagination.preparePendingBlock(sentence, {
+                includeUnrenderedHistory: true
+            });
         } else {
             document.dispatchEvent(new CustomEvent('book-pagination:prepare-block', {
                 detail: {
@@ -1110,23 +1127,39 @@ class UIDisplayHandlerModule extends BaseModule {
     waitForWebGLPageFlip(detail = {}) {
         return new Promise((resolve) => {
             let resolved = false;
-            const finish = () => {
+            const cleanup = () => {
+                window.clearTimeout(timeout);
+                document.removeEventListener('webgl-book:page-flip-started', onStarted);
+                document.removeEventListener('webgl-book:page-flip-finished', onFinished);
+            };
+            const finish = (result) => {
                 if (resolved) return;
                 resolved = true;
-                window.clearTimeout(timeout);
-                document.removeEventListener('webgl-book:page-flip-finished', finish);
-                resolve(true);
+                cleanup();
+                resolve(result);
             };
-            const timeout = window.setTimeout(finish, 1400);
-            document.addEventListener('webgl-book:page-flip-finished', finish, { once: true });
+            const requestedTargetSpread = Number.isFinite(Number(detail.targetSpread))
+                ? Math.max(0, Math.round(Number(detail.targetSpread)))
+                : null;
+            const matchesTarget = (eventDetail = {}) => requestedTargetSpread == null
+                || Math.max(0, Math.round(Number(eventDetail.targetSpread || 0))) === requestedTargetSpread;
+            const onStarted = (event) => {
+                if (!matchesTarget(event.detail || {})) return;
+                document.documentElement.dataset.webglLastStartedPageFlip = JSON.stringify(event.detail || {});
+            };
+            const onFinished = (event) => {
+                if (!matchesTarget(event.detail || {})) return;
+                finish(true);
+            };
+            const timeout = window.setTimeout(() => finish(false), 2400);
+            document.addEventListener('webgl-book:page-flip-started', onStarted);
+            document.addEventListener('webgl-book:page-flip-finished', onFinished);
             document.dispatchEvent(new CustomEvent('webgl-book:request-page-flip', {
                 detail: {
                     direction: Math.sign(Number(detail.direction || 1)) || 1,
                     reason: detail.reason || 'pending-block-overflow',
                     force: true,
-                    targetSpread: Number.isFinite(Number(detail.targetSpread))
-                        ? Math.max(0, Math.round(Number(detail.targetSpread)))
-                        : null
+                    targetSpread: requestedTargetSpread
                 }
             }));
         });

public/js/webgl-book-lab.js

@@ -204,6 +204,7 @@ let bookPaginationState = {
     spreadCount: 1,
     writtenPageLimit: 0
 };
+let maxVisitedPagePosition = 0;
 const normalFlipDuration = 900;
 const fastFlipDuration = 520;
 const fastFlipCount = 10;
@@ -364,7 +365,7 @@ const materials = {
         emissive: 0x100d08,
         emissiveIntensity: 0.004,
         envMapIntensity: 0.01,
-        side: THREE.DoubleSide
+        side: THREE.FrontSide
     }),
     leftPage: new THREE.MeshStandardMaterial({
         color: 0xffffff,
@@ -414,7 +415,7 @@ const materials = {
 };
 materials.flipPageBackSurface = materials.flipPageSurface.clone();
 materials.flipPageBackSurface.map = getBlankPageTexture();
-materials.flipPageBackSurface.side = THREE.DoubleSide;
+materials.flipPageBackSurface.side = THREE.FrontSide;
 materials.flipPageEdge = materials.pageSurface.clone();
 materials.flipPageEdge.map = paperTextures.edge;
 materials.flipPageEdge.normalMap = paperTextures.normal;
@@ -531,6 +532,7 @@ window.BookLabDebug = {
             pageReserve,
             progress: readingProgress,
             pagePosition: getCurrentPagePosition(),
+            maxVisitedPagePosition,
             spreadIndex: bookPaginationState.spreadIndex,
             writtenPageLimit: bookPaginationState.writtenPageLimit
         };
@@ -541,10 +543,17 @@ window.BookLabDebug = {
             spreadCount: Math.max(1, Number(state.spreadCount ?? bookPaginationState.spreadCount ?? 1)),
             writtenPageLimit: Math.max(0, Number(state.writtenPageLimit ?? bookPaginationState.writtenPageLimit ?? 0))
         };
+        maxVisitedPagePosition = Math.max(maxVisitedPagePosition, getCurrentPagePosition());
         growBookIfWritableLimitReached();
         syncBookControls();
         return this.getBookState();
     },
+    setMaxVisitedPagePosition(value) {
+        const page = Math.max(0, Math.round(Number(value || 0)));
+        maxVisitedPagePosition = Math.max(maxVisitedPagePosition, page);
+        syncBookControls();
+        return maxVisitedPagePosition;
+    },
     navigateToPagePosition(value) {
         return navigateToPagePosition(value);
     },
@@ -616,9 +625,26 @@ document.addEventListener('webgl-book:page-cache-problem', (event) => {
 });
 document.addEventListener('book-pagination:spread-updated', (event) => {
     const detail = event.detail || {};
+    const incomingSpreadIndex = Math.max(0, Number(detail.spreadIndex || 0));
+    const latestBlockId = Math.max(0, Number(detail.latestBlockId || 0));
+    const latestRenderedBlockId = Math.max(0, Number(detail.latestRenderedBlockId || 0));
+    if (
+        latestBlockId > latestRenderedBlockId
+        && detail.allowFutureUnrendered !== true
+        && activeFlips.length === 0
+        && incomingSpreadIndex > Math.max(0, Number(bookPaginationState.spreadIndex || 0))
+    ) {
+        markPageTextureTiming('spreadUpdate:deferred-future-unrendered', {
+            incomingSpreadIndex,
+            visibleSpreadIndex: bookPaginationState.spreadIndex,
+            latestBlockId,
+            latestRenderedBlockId
+        });
+        return;
+    }
     const previousPageCount = bookPageCount;
     bookPaginationState = {
-        spreadIndex: Math.max(0, Number(detail.spreadIndex || 0)),
+        spreadIndex: incomingSpreadIndex,
         spreadCount: Math.max(1, Number(detail.spreadCount || 1)),
         writtenPageLimit: Math.max(0, Number(detail.writtenPageLimit || 0))
     };
@@ -1920,10 +1946,10 @@ function ensureBottomNavigation() {
     startButton.addEventListener('click', () => navigateToPagePosition(0));
     backButton.addEventListener('click', () => navigateByPageDelta(-1));
     forwardButton.addEventListener('click', () => navigateByPageDelta(1));
-    endButton.addEventListener('click', () => navigateToPagePosition(bookPaginationState.writtenPageLimit));
+    endButton.addEventListener('click', () => navigateToPagePosition(maxVisitedPagePosition));
     slider.addEventListener('input', () => {
         const requested = Number(slider.value);
-        const clamped = Math.min(requested, Math.max(0, bookPaginationState.writtenPageLimit || 0), getWritablePageLimit());
+        const clamped = Math.min(requested, maxVisitedPagePosition, getWritablePageLimit());
         if (requested !== clamped) slider.value = String(clamped);
         pageLabel.textContent = `${appInitialState.t?.('webgl.page') || 'Page'} ${clamped}`;
     });
@@ -1952,8 +1978,7 @@ function navigateByPageDelta(delta) {
 
 function navigateToPagePosition(pagePosition) {
     const writableLimit = getWritablePageLimit();
-    const writtenLimit = Math.max(0, bookPaginationState.writtenPageLimit || 0);
-    const targetPage = THREE.MathUtils.clamp(Math.round(Number(pagePosition || 0)), 0, Math.min(writableLimit, writtenLimit));
+    const targetPage = THREE.MathUtils.clamp(Math.round(Number(pagePosition || 0)), 0, Math.min(writableLimit, maxVisitedPagePosition));
     const currentPage = getCurrentPagePosition();
     if (targetPage === currentPage) {
         syncBookControls();
@@ -1984,9 +2009,8 @@ function syncBookControls() {
 function syncBottomNavigation() {
     if (!bottomNavigation) return;
     const currentPage = getCurrentPagePosition();
-    const writtenLimit = Math.max(0, bookPaginationState.writtenPageLimit || 0);
     const writableLimit = getWritablePageLimit();
-    const navigableLimit = Math.min(writtenLimit, writableLimit);
+    const navigableLimit = Math.min(maxVisitedPagePosition, writableLimit);
     const reservedStart = Math.max(0, writableLimit);
     bottomNavigation.slider.max = String(Math.max(0, bookPageCount));
     bottomNavigation.slider.value = String(Math.min(currentPage, navigableLimit));
@@ -1994,7 +2018,7 @@ function syncBottomNavigation() {
     bottomNavigation.maxLabel.textContent = String(bookPageCount);
     bottomNavigation.pageLabel.textContent = `${appInitialState.t?.('webgl.page') || 'Page'} ${Math.min(currentPage, navigableLimit)}`;
     bottomNavigation.root.style.setProperty('--book-nav-position', `${bookPageCount > 0 ? currentPage / bookPageCount : 0}`);
-    bottomNavigation.root.style.setProperty('--book-nav-written', `${bookPageCount > 0 ? writtenLimit / bookPageCount : 0}`);
+    bottomNavigation.root.style.setProperty('--book-nav-written', `${bookPageCount > 0 ? maxVisitedPagePosition / bookPageCount : 0}`);
     bottomNavigation.root.style.setProperty('--book-nav-reserve-start', `${bookPageCount > 0 ? reservedStart / bookPageCount : 1}`);
     bottomNavigation.root.dataset.bookSize = String(bookPageCount);
     bottomNavigation.root.dataset.pageReserve = String(pageReserve);
@@ -2132,7 +2156,7 @@ function recordPageCacheProblem(detail = {}) {
 function rememberResidentPageTexture(pageMeta = null, texture = null, sourceCanvas = null, ownsTexture = true) {
     const pageIndex = Number(pageMeta?.pageIndex);
     if (!texture || !Number.isFinite(pageIndex) || pageIndex < 0) return null;
-    const key = makePageMetaForCache(pageIndex).pageIndex;
+    const key = makeResidentPageTextureKey(pageMeta);
     const existing = residentPageTextures.get(key);
     if (isOlderPageTextureMeta(pageMeta, existing?.pageMeta)) return existing?.texture || null;
     if (existing?.ownsTexture && existing.texture && existing.texture !== texture) existing.texture.dispose?.();
@@ -2167,13 +2191,26 @@ function isOlderPageTextureMeta(incoming = {}, existing = null) {
 }
 
 function makePageMetaForCache(pageIndex) {
+    const index = Math.max(0, Math.round(Number(pageIndex || 0)));
+    const paginationMeta = getPaginationPageMeta(index) || {};
     return {
-        pageIndex: Math.max(0, Math.round(Number(pageIndex || 0))),
+        ...paginationMeta,
+        pageIndex: index,
         width: pageTextureWidth,
         height: leftCanvas?.height || Math.round(pageTextureWidth * PROCEDURAL_BOOK.PAGE_DEPTH / PROCEDURAL_BOOK.PAGE_WIDTH)
     };
 }
 
+function makeResidentPageTextureKey(pageMetaOrIndex = {}) {
+    const pageMeta = typeof pageMetaOrIndex === 'number'
+        ? makePageMetaForCache(pageMetaOrIndex)
+        : pageMetaOrIndex || {};
+    const pageIndex = Math.max(0, Math.round(Number(pageMeta.pageIndex || 0)));
+    const kind = String(pageMeta.kind || 'content');
+    const section = String(pageMeta.section || 'body');
+    return `${pageIndex}:${kind}:${section}`;
+}
+
 function spreadPageIndices(spreadIndex) {
     const spread = Math.max(0, Math.round(Number(spreadIndex || 0)));
     return {
@@ -2183,7 +2220,7 @@ function spreadPageIndices(spreadIndex) {
 }
 
 function getResidentPageTexture(pageIndex) {
-    const key = makePageMetaForCache(pageIndex).pageIndex;
+    const key = makeResidentPageTextureKey(pageIndex);
     const resident = residentPageTextures.get(key);
     if (!resident) return null;
     resident.lastUsedAt = performance.now();
@@ -2195,7 +2232,7 @@ function getResidentPageTexture(pageIndex) {
 function getResidentPageTextureForMeta(pageMeta = null) {
     const pageIndex = Number(pageMeta?.pageIndex);
     if (!Number.isFinite(pageIndex)) return null;
-    const key = makePageMetaForCache(pageIndex).pageIndex;
+    const key = makeResidentPageTextureKey(pageMeta);
     const resident = residentPageTextures.get(key);
     if (!resident || isOlderPageTextureMeta(pageMeta, resident.pageMeta)) return null;
     return getResidentPageTexture(pageIndex);
@@ -2203,13 +2240,20 @@ function getResidentPageTextureForMeta(pageMeta = null) {
 
 async function preloadCachedPageTexture(pageIndex) {
     const meta = makePageMetaForCache(pageIndex);
-    if (residentPageTextures.has(meta.pageIndex)) {
+    const residentKey = makeResidentPageTextureKey(meta);
+    if (residentPageTextures.has(residentKey)) {
         getResidentPageTexture(meta.pageIndex);
-        return residentPageTextures.get(meta.pageIndex)?.texture || null;
+        return residentPageTextures.get(residentKey)?.texture || null;
     }
     const cache = window.WebGLPageCache || window.moduleRegistry?.getModule?.('webgl-page-cache') || null;
     const sourceCanvas = await cache?.getPageCanvas?.(meta);
     if (!sourceCanvas) {
+        const pageMeta = getPaginationPageMeta(meta.pageIndex);
+        if (pageMeta?.kind === 'blank') {
+            const blankTexture = getBlankPageTexture();
+            rememberResidentPageTexture({ ...meta, ...pageMeta }, blankTexture, null, false);
+            return blankTexture;
+        }
         recordPageCacheProblem({
             type: 'db-cache-miss',
             pageIndex: meta.pageIndex,
@@ -2220,7 +2264,7 @@ async function preloadCachedPageTexture(pageIndex) {
     }
     const texture = createPageCanvasTexture(sourceCanvas);
     const cachedMeta = sourceCanvas.__webglPageCacheMeta || meta;
-    residentPageTextures.set(meta.pageIndex, {
+    residentPageTextures.set(residentKey, {
         texture,
         sourceCanvas,
         lastUsedAt: performance.now(),
@@ -2236,6 +2280,19 @@ async function preloadCachedPageTexture(pageIndex) {
     return texture;
 }
 
+function getPaginationPageMeta(pageIndex) {
+    const index = Math.max(0, Math.round(Number(pageIndex || 0)));
+    const spreadIndex = Math.floor(index / 2);
+    const side = index % 2 === 0 ? 'left' : 'right';
+    const pagination = window.moduleRegistry?.getModule?.('book-pagination') || null;
+    const spread = typeof pagination?.getSpread === 'function'
+        ? pagination.getSpread(spreadIndex)
+        : Array.isArray(pagination?.spreads)
+            ? pagination.spreads[spreadIndex]
+            : null;
+    return spread?.pageMeta?.[side] || null;
+}
+
 async function prewarmSpreadTextures(spreadIndex) {
     const indices = spreadPageIndices(spreadIndex);
     const [left, right] = await Promise.all([
@@ -2276,7 +2333,8 @@ function takePreparedPageTexture(side, revealDetail = {}) {
 function uploadPageTextureDirect(side, sourceCanvas, pageMeta = null) {
     const texture = side === 'left' ? leftTexture : rightTexture;
     const material = side === 'left' ? materials.leftPage : materials.rightPage;
-    const residentTexture = Number.isFinite(Number(pageMeta?.pageIndex))
+    const shouldUseResidentTexture = pageMeta?.kind !== 'title';
+    const residentTexture = shouldUseResidentTexture && Number.isFinite(Number(pageMeta?.pageIndex))
         ? getResidentPageTextureForMeta(pageMeta)
         : null;
     markPageTextureTiming('directUpload:start', {
@@ -2703,6 +2761,13 @@ function startPageFlipPrepared(direction, options = {}) {
     delete document.documentElement.dataset.webglPendingPageFlip;
     activeFlips.push(flip);
     setPageFlipActiveFlag();
+    document.dispatchEvent(new CustomEvent('webgl-book:page-flip-started', {
+        detail: {
+            direction: flip.direction,
+            sourceSide: flip.sourcePageSide || (flip.direction > 0 ? 'right' : 'left'),
+            targetSpread: Number.isFinite(Number(flip.targetSpread)) ? Math.max(0, Math.round(Number(flip.targetSpread))) : null
+        }
+    }));
     syncBookControls();
     updateActiveFlips(flip.startTime);
     return true;
@@ -2723,6 +2788,7 @@ function startFastPageFlipPrepared(direction, options = {}) {
     if (activeFlips.length || !currentProceduralBookModel || !canPageFlip(direction)) return false;
     const firstFlip = createPageFlip(direction, performance.now(), fastFlipDuration);
     if (!firstFlip) return false;
+    firstFlip.targetSpread = Number.isFinite(Number(options.targetSpread)) ? Math.max(0, Math.round(Number(options.targetSpread))) : null;
     if (!prepareStaticPageForFlip(firstFlip, options.prewarm || null)) return false;
     const startTime = firstFlip.startTime;
     const interval = fastFlipDuration / fastFlipOverlap;
@@ -2740,6 +2806,14 @@ function startFastPageFlipPrepared(direction, options = {}) {
         });
     }
     setPageFlipActiveFlag();
+    document.dispatchEvent(new CustomEvent('webgl-book:page-flip-started', {
+        detail: {
+            direction: firstFlip.direction,
+            sourceSide: firstFlip.sourcePageSide || (firstFlip.direction > 0 ? 'right' : 'left'),
+            targetSpread: Number.isFinite(Number(firstFlip.targetSpread)) ? Math.max(0, Math.round(Number(firstFlip.targetSpread))) : null,
+            fast: true
+        }
+    }));
     syncBookControls();
     updateActiveFlips(startTime);
     return true;
@@ -2774,7 +2848,8 @@ function prepareStaticPageForFlip(flip, prewarm = null) {
         : Math.max(0, Math.round(Number(bookPaginationState.spreadIndex || 0)) + Math.sign(Number(flip.direction || 0)));
     const targetPages = spreadPageIndices(targetSpread);
     const targetBackPageIndex = flip.direction > 0 ? targetPages.left : targetPages.right;
-    const residentBackTexture = getResidentPageTexture(targetBackPageIndex);
+    const prewarmedBackTexture = flip.direction > 0 ? prewarm?.next?.left : prewarm?.next?.right;
+    const residentBackTexture = prewarmedBackTexture || getResidentPageTexture(targetBackPageIndex);
     const requiresWrittenTexture = targetBackPageIndex <= Math.max(2, Number(bookPaginationState.writtenPageLimit || 0));
     if (!residentBackTexture && requiresWrittenTexture) {
         recordPageCacheProblem({
@@ -2827,7 +2902,7 @@ function prepareStaticPageForFlip(flip, prewarm = null) {
 function canPageFlip(direction) {
     if (!currentProceduralBookModel) return false;
     const currentPage = getCurrentPagePosition();
-    const maxNavigablePage = Math.min(Math.max(0, bookPaginationState.writtenPageLimit || 0), getWritablePageLimit());
+    const maxNavigablePage = Math.min(maxVisitedPagePosition, getWritablePageLimit());
     if (direction > 0) return currentPage < maxNavigablePage;
     return currentPage > 0;
 }
@@ -3031,7 +3106,7 @@ function lineYAtX(points, x) {
 
 function setActivePageGeometry(flip, surface) {
     if (!flip.mesh) {
-        const geometry = createFlippingPageGeometry(surface);
+        const geometry = createFlippingPageGeometry(surface, flip.direction);
         flip.mesh = new THREE.Mesh(geometry, [
             materials.flipPageSurface,
             materials.flipPageBackSurface,
@@ -3045,13 +3120,13 @@ function setActivePageGeometry(flip, surface) {
         return;
     }
     if (!updateFlippingPageGeometry(flip.mesh.geometry, surface)) {
-        const geometry = createFlippingPageGeometry(surface);
+        const geometry = createFlippingPageGeometry(surface, flip.direction);
         flip.mesh.geometry.dispose();
         flip.mesh.geometry = geometry;
     }
 }
 
-function createFlippingPageGeometry(surface) {
+function createFlippingPageGeometry(surface, direction = 1) {
     const positions = [];
     const uvs = [];
     const indices = [];
@@ -3063,10 +3138,12 @@ function createFlippingPageGeometry(surface) {
     const pageThickness = Math.max(0.0008, Number(PROCEDURAL_BOOK.SHEET_THICKNESS_MODEL || 0.001));
     const widthSegments = surface.length - 1;
     const depthSegments = surface[0].length - 1;
-    const push = (point, yOffset, u, v) => {
+    const sourceSide = direction > 0 ? 1 : -1;
+    const targetSide = -sourceSide;
+    const push = (point, yOffset, uv) => {
         const index = positions.length / 3;
         positions.push(point.x, point.y + yOffset, point.z);
-        uvs.push(u, v);
+        uvs.push(uv.x, uv.y);
         return index;
     };
 
@@ -3076,8 +3153,8 @@ function createFlippingPageGeometry(surface) {
         const u = widthSegments <= 0 ? 0 : widthIndex / widthSegments;
         rowPoints.forEach((point, depthIndex) => {
             const v = depthSegments <= 0 ? 0 : depthIndex / depthSegments;
-            topRow.push(push(point, pageThickness, u, v));
-            bottomRow.push(push(point, 0, u, 1 - v));
+            topRow.push(push(point, pageThickness, pageUvForSide(sourceSide, u, v)));
+            bottomRow.push(push(point, 0, pageUvForSide(targetSide, u, v)));
         });
         topGrid.push(topRow);
         bottomGrid.push(bottomRow);
@@ -3123,6 +3200,13 @@ function createFlippingPageGeometry(surface) {
     }
 }
 
+function pageUvForSide(side, u, v) {
+    return {
+        x: side < 0 ? 1 - u : u,
+        y: 1 - v
+    };
+}
+
 function updateFlippingPageGeometry(geometry, surface) {
     const position = geometry?.getAttribute?.('position');
     if (!position || !surface?.length || !surface[0]?.length) return false;
@@ -3160,6 +3244,7 @@ function finishActiveFlip(flip) {
             ...bookPaginationState,
             spreadIndex: Math.max(0, Math.round(Number(flip.targetSpread)))
         };
+        maxVisitedPagePosition = Math.max(maxVisitedPagePosition, getCurrentPagePosition());
         syncReadingProgressToCurrentPage();
     }
     document.dispatchEvent(new CustomEvent('webgl-book:page-flip-finished', {

public/js/webgl-book-scene-module.js

@@ -351,6 +351,10 @@ class WebGLBookSceneModule extends BaseModule {
                     this.persistenceManager?.updatePreference?.('webgl', 'bookProgress', progress);
                     window.BookLabDebug?.setReadingProgress?.(progress);
                 }
+                const maxVisitedPagePosition = Number(state.maxVisitedPagePosition ?? state.pagePosition);
+                if (Number.isFinite(maxVisitedPagePosition)) {
+                    window.BookLabDebug?.setMaxVisitedPagePosition?.(maxVisitedPagePosition);
+                }
             }
         };
     }

public/js/webgl-page-cache-module.js

@@ -84,11 +84,13 @@ class WebGLPageCacheModule extends BaseModule {
         return this.db.transaction([this.storeName], mode).objectStore(this.storeName);
     }
 
-    makePageKey({ pageIndex, width, height, cacheKey = window.MODULE_CACHE_BUSTER || 'dev' } = {}) {
+    makePageKey({ pageIndex, width, height, kind = 'content', section = 'body', cacheKey = window.MODULE_CACHE_BUSTER || 'dev' } = {}) {
         const safePage = Math.max(0, Math.round(Number(pageIndex || 0)));
         const safeWidth = Math.max(1, Math.round(Number(width || 0)));
         const safeHeight = Math.max(1, Math.round(Number(height || 0)));
-        return `${cacheKey}:page:${safePage}:${safeWidth}x${safeHeight}`;
+        const safeKind = String(kind || 'content').replace(/[^a-z0-9_-]/gi, '');
+        const safeSection = String(section || 'body').replace(/[^a-z0-9_-]/gi, '');
+        return `${cacheKey}:page:${safePage}:${safeKind}:${safeSection}:${safeWidth}x${safeHeight}`;
     }
 
     async cachePageCanvas(pageMeta = {}, canvas = null) {
@@ -99,6 +101,8 @@ class WebGLPageCacheModule extends BaseModule {
             pageIndex,
             width: canvas.width,
             height: canvas.height,
+            kind: pageMeta.kind,
+            section: pageMeta.section,
             cacheKey: pageMeta.cacheKey
         });
         try {
@@ -119,6 +123,8 @@ class WebGLPageCacheModule extends BaseModule {
                     height: canvas.height,
                     contentVersion: Math.max(0, Number(pageMeta.contentVersion || 0)),
                     completenessScore: Math.max(0, Number(pageMeta.completenessScore || 0)),
+                    kind: pageMeta.kind || 'content',
+                    section: pageMeta.section || 'body',
                     maxBlockId: Math.max(0, Number(pageMeta.maxBlockId || 0)),
                     lineCount: Math.max(0, Number(pageMeta.lineCount || 0)),
                     blob,
@@ -168,6 +174,8 @@ class WebGLPageCacheModule extends BaseModule {
             const canvas = await this.blobToCanvas(entry.blob, entry.width, entry.height);
             if (canvas) canvas.__webglPageCacheMeta = {
                 pageIndex: entry.pageIndex,
+                kind: entry.kind || pageMeta.kind || 'content',
+                section: entry.section || pageMeta.section || 'body',
                 contentVersion: entry.contentVersion,
                 completenessScore: entry.completenessScore,
                 maxBlockId: entry.maxBlockId,

scripts/check-webgl-book-lab.js

@@ -163,7 +163,7 @@ const checks = [
     ['webgl page text textures avoid mipmap generation', /function configurePageCanvasTexture/.test(source) && /texture\.minFilter = THREE\.LinearFilter/.test(source) && /texture\.generateMipmaps = false/.test(source)],
     ['webgl reveal shader masks against a base-page texture instead of flat color blocks', /bookRevealBaseMap/.test(source) && /bookRevealUseBaseMap/.test(source) && /revealBaseColor/.test(source) && /baseCanvas/.test(textureRendererSource)],
     ['webgl reveal shader masks antialiased ink and uses smooth line-dominant scan', /smoothstep\(0\.52, 0\.9, luminance\)/.test(source) && /local\.x \* 0\.96/.test(source) && /bookRevealSoftness = \{ value: 0\.025 \}/.test(source)],
-    ['webgl reveal line timings use absolute block word timing across split pages', /assignRevealTiming/.test(textureRendererSource) && /wordStart/.test(textureRendererSource) && /blockWordStart/.test(textureRendererSource) && /wordTimings\[wordStart\]/.test(textureRendererSource) && /durationMs: sideRegions\.reduce/.test(textureRendererSource)],
+    ['webgl reveal line timings use global area timing across split-page spreads', /assignRevealTiming/.test(textureRendererSource) && /sourceSpreads/.test(textureRendererSource) && /this\.pagination\?\.spreads/.test(textureRendererSource) && /spreadIndex/.test(textureRendererSource) && /totalDuration \* \(Math\.max\(1, region\.area\) \/ totalArea\)/.test(textureRendererSource) && /durationMs: sideRegions\.reduce/.test(textureRendererSource)],
     ['webgl page format reduces only outer margins from previous value', /outerBaseIn: 0\.27/.test(pageFormatSource) && /outerThicknessFactor: 0\.015/.test(pageFormatSource) && /outerMaxIn: 0\.315/.test(pageFormatSource) && /innerBaseIn: 0\.42/.test(pageFormatSource)],
     ['webgl mode enlarges and inverts DOM overlay text without touching 2D mode', /body\.webgl-mode \{[\s\S]*font-size: 18px;/.test(styleSource) && /body\.webgl-mode \.choice-list \.choice-button/.test(styleSource) && /rgba\(246, 231, 201/.test(styleSource)],
     ['webgl choice overlay hides title clutter and prevents horizontal scrollbar', /body\.webgl-mode #page_left #game_title/.test(styleSource) && /body\.webgl-mode #page_left #start_prompt/.test(styleSource) && /overflow-x: hidden/.test(styleSource) && /book\.style\.width = 'min\(44rem/.test(webglSceneSource)],
@@ -175,12 +175,13 @@ const checks = [
     ['webgl scene does not republish 3D page textures from DOM refresh events', !/addEventListener\(document, 'story:turn-start', this\.triggerTextureRefresh\)/.test(webglSceneSource) && !/addEventListener\(document, 'story:turn-complete', this\.triggerTextureRefresh\)/.test(webglSceneSource) && !/addEventListener\(document, 'story:history-updated', this\.triggerTextureRefresh\)/.test(webglSceneSource) && !/addEventListener\(document, 'input', this\.triggerTextureRefresh/.test(webglSceneSource) && !/addEventListener\(document, 'change', this\.triggerTextureRefresh/.test(webglSceneSource)],
     ['webgl scene adoptPageContent does not republish 3D page textures', !/triggerTextureRefresh/.test(methodBody(webglSceneSource, 'adoptPageContent'))],
     ['webgl book starts at progress zero', /const DEFAULT_BOOK_PROGRESS = 0;/.test(webglSceneSource) && /appInitialState\.progress \?\? '0'/.test(source)],
-    ['pagination opens with blank left and title right spread', /this\.createBlankPage\(0, \{ section: 'frontmatter' \}\)/.test(bookPaginationSource) && /this\.createTitlePage\(1\)/.test(bookPaginationSource) && /this\.createBlankPage\(2, \{ section: 'frontmatter' \}\)/.test(bookPaginationSource)],
+    ['pagination opens with blank left and title right spread', /this\.createBlankPage\(0, \{ section: 'frontmatter' \}\)/.test(bookPaginationSource) && /this\.createTitlePage\(1\)/.test(bookPaginationSource) && /this\.createBlankPage\(2, \{ section: 'frontmatter' \}\)/.test(bookPaginationSource) && /this\.pages = this\.buildPages\(\[\]\);/.test(bookPaginationSource) && /this\.currentSpreadIndex = 0;[\s\S]*this\.publish\(\{ reason: 'initial-title-spread', allowFutureUnrendered: true \}\);/.test(bookPaginationSource)],
+    ['texture renderer adopts initial pagination spread so title page is painted after loader order', /this\.currentSpread = this\.pagination\?\.getCurrentSpread\?\.\(\) \|\| \{ index: 0/.test(textureRendererSource) && /this\.drawSpread\(this\.currentSpread\);/.test(textureRendererSource)],
     ['pagination publishes page metadata and advances near the end of physical flips', /pageMeta/.test(bookPaginationSource) && /webgl-book:page-flip-near-end/.test(bookPaginationSource) && /this\.setCurrentSpread\(this\.currentSpreadIndex \+ direction\)/.test(bookPaginationSource)],
     ['texture renderer draws title page and page numbers from versioned page metadata', /drawTitlePage/.test(textureRendererSource) && /game_title/.test(textureRendererSource) && /drawPageNumber/.test(textureRendererSource) && /pageMeta: this\.buildPublishPageMeta\(sidesToPublish\)/.test(textureRendererSource)],
     ['texture renderer uses plural page margin metrics for page numbers', /this\.metrics\.margins\.bottom/.test(textureRendererSource) && !/this\.metrics\.margin\.bottom/.test(textureRendererSource)],
     ['webgl flip borrows resident page texture and blanks right stack before forward animation', /prepareStaticPageForFlip/.test(source) && /materials\.flipPageSurface\.map = sourceTexture/.test(source) && /materials\.rightPage\.map = blankTexture/.test(source) && /webgl-book:page-flip-near-end/.test(source)],
-    ['webgl flip never falls back to the opposite visible stack for target back texture', /const residentBackTexture = getResidentPageTexture\(targetBackPageIndex\)/.test(source) && /const backTexture = residentBackTexture \|\| getBlankPageTexture\(\)/.test(source) && !/oppositeMaterial\?\.map/.test(methodBody(source, 'prepareStaticPageForFlip'))],
+    ['webgl flip never falls back to the opposite visible stack for target back texture', /const prewarmedBackTexture = flip\.direction > 0 \? prewarm\?\.next\?\.left : prewarm\?\.next\?\.right/.test(source) && /const residentBackTexture = prewarmedBackTexture \|\| getResidentPageTexture\(targetBackPageIndex\)/.test(source) && /const backTexture = residentBackTexture \|\| getBlankPageTexture\(\)/.test(source) && !/oppositeMaterial\?\.map/.test(methodBody(source, 'prepareStaticPageForFlip'))],
     ['webgl page canvas metadata accepts explicit blank sides instead of retaining stale pages', /hasLeftMeta/.test(source) && /hasRightMeta/.test(source) && /Object\.prototype\.hasOwnProperty\.call\(detail\.pageMeta, 'right'\)/.test(source)],
     ['texture renderer publishes both spread sides for reveal preparation and fallback start', /this\.drawSpread\(this\.currentSpread \|\| this\.pagination\?\.getCurrentSpread\?\.\(\), \['left', 'right'\]\)/.test(textureRendererSource) && /const sides = \['left', 'right'\]/.test(textureRendererSource) && /pageMeta: prepared\.pageMeta \|\| \{\}/.test(textureRendererSource)],
     ['texture renderer marks page canvases with content versions before cache writes', /pageContentVersions/.test(textureRendererSource) && /buildPublishPageMeta/.test(textureRendererSource) && /completenessScore: \(maxBlockId \* 1000\) \+ lineCount/.test(textureRendererSource)],
@@ -188,7 +189,7 @@ const checks = [
     ['webgl resident page texture cache rejects older page versions before direct reuse', /isOlderPageTextureMeta/.test(source) && /getResidentPageTextureForMeta/.test(source) && /getResidentPageTextureForMeta\(pageMeta\)/.test(source)],
     ['webgl flip page is a two-sided body using paper-thickness constants', /flipPageBackSurface/.test(source) && /flipPageEdge/.test(source) && /new THREE\.Mesh\(geometry, \[\s*materials\.flipPageSurface,\s*materials\.flipPageBackSurface,\s*materials\.flipPageEdge\s*\]\)/.test(source) && /PROCEDURAL_BOOK\.SHEET_THICKNESS_MODEL/.test(source) && /geometry\.addGroup\(0, topIndices\.length, 0\)/.test(source)],
     ['webgl animated page front and back maps are independently switchable before animation starts', /materials\.flipPageBackSurface = materials\.flipPageSurface\.clone\(\)/.test(source) && /materials\.flipPageSurface\.map = sourceTexture/.test(source) && /materials\.flipPageBackSurface\.map = backTexture/.test(source) && /flip\.sourceTexture = sourceTexture/.test(source) && /flip\.backTexture = backTexture/.test(source)],
-    ['webgl animated page back face uses its own unflipped page orientation', /bottomRow\.push\(push\(point, 0, u, 1 - v\)\)/.test(source)],
+    ['webgl animated page back face uses destination-side page UV orientation', /pageUvForSide\(targetSide, u, v\)/.test(source) && /pageUvForSide\(sourceSide, u, v\)/.test(source) && /side < 0 \? 1 - u : u/.test(source)],
     ['webgl animated page reuses geometry buffers during flips', /function updateFlippingPageGeometry/.test(source) && /position\.needsUpdate = true/.test(source) && /updateFlippingPageGeometry\(flip\.mesh\.geometry, surface\)/.test(source) && !/flip\.mesh\.geometry\.dispose\(\);\s*flip\.mesh\.geometry = geometry;/.test(methodBody(source, 'setActivePageGeometry'))],
     ['webgl scene targets 60fps with browser-frame scheduling live mirror and static heavy refresh', /const targetFrameDurationMs = 1000 \/ 60/.test(source) && /this\.targetFrameDurationMs = 1000 \/ 60/.test(textureRendererSource) && /requestAnimationFrame\(animate\)/.test(source) && /const refreshStaticSceneBuffers = staticSceneBuffersDirty \|\| activeFlips\.length > 0/.test(source) && /updateTableReflection\(\);/.test(source) && /mirrorRefreshesEveryFrame: true/.test(source) && !/setTimeout\(animate/.test(source)],
     ['webgl scene lowers mirror target and caps table film maps to 2k', /const reflectionPixelRatio = 1/.test(source) && /const tableReflectionBaseWidth = 2048/.test(source) && /const tableReflectionBaseHeight = 1152/.test(source) && /tableDustTexture = loadUtilityTexture\('\/assets\/webgl\/table_dust_4k\.png', \{ maxSize: 2048 \}\)/.test(source) && /tableGreaseTexture = loadUtilityTexture\('\/assets\/webgl\/table_grease_4k\.png', \{ maxSize: 2048 \}\)/.test(source)],
@@ -205,7 +206,12 @@ const checks = [
     ['webgl right-page completion arms a durable autoplay-targeted flip without bypassing choices', /handleRevealCommittedForPageFlip/.test(source) && /tryStartPendingRightPageFlip/.test(source) && /pendingRightPageFlipAutoplay/.test(source) && /const targetSpread = Math\.max\(0, Math\.round\(Number\(bookPaginationState\.spreadIndex \|\| 0\)\) \+ 1\)/.test(source) && /force: options\.force === true \|\| pendingRightPageFlipAutoplay/.test(source) && /isChoiceAwaitingPlayer/.test(source) && /pendingRightPageFlip = true/.test(source)],
     ['webgl reveal clock follows absolute playback time and continues across page flips', /activeRevealBlockStarts/.test(source) && /state\.visualElapsedMs = Math\.max\(0, now - state\.startedAt\)/.test(source) && !/Math\.min\(revealFrameDeltaMs, targetFrameDurationMs\)/.test(source) && /prewarmFlipTextures\(1, targetSpread\)/.test(source)],
     ['texture renderer preloads every spread touched by an active reveal block', /preloadAdditionalRevealSpreads/.test(textureRendererSource) && /spreadContainsBlock/.test(textureRendererSource) && /this\.drawSpread\(spread, \['left', 'right'\], \{ preloadOnly: true \}\)/.test(textureRendererSource) && /this\.activeAnimations\.has\(id\)/.test(textureRendererSource)],
+    ['webgl visible spread state ignores future unrendered prepare publishes before flip', /spreadUpdate:deferred-future-unrendered/.test(source) && /incomingSpreadIndex > Math\.max\(0, Number\(bookPaginationState\.spreadIndex/.test(source) && /this\.drawSpread\(this\.currentSpread, \['left', 'right'\], \{ preloadOnly: true \}\)/.test(textureRendererSource)],
+    ['3D overflow reveal preloads target spread before forced page flip', /previewRevealDetail/.test(uiDisplayHandlerSource) && /preloadOnly: true/.test(uiDisplayHandlerSource) && /bookTextureRenderer\.prepareRevealBlock\(previewRevealDetail, \{ preloadOnly: true \}\)/.test(uiDisplayHandlerSource) && /await this\.waitForWebGLPageFlip/.test(uiDisplayHandlerSource)],
+    ['webgl navigation buttons use visited page limit instead of future prepared pages', /maxVisitedPagePosition/.test(source) && /navigateToPagePosition\(maxVisitedPagePosition\)/.test(source) && /const navigableLimit = Math\.min\(maxVisitedPagePosition, writableLimit\)/.test(source) && !/navigateToPagePosition\(bookPaginationState\.writtenPageLimit\)/.test(source)],
+    ['webgl save restore carries visited page limit for navigation', /maxVisitedPagePosition/.test(source) && /setMaxVisitedPagePosition/.test(source) && /state\.maxVisitedPagePosition \?\? state\.pagePosition/.test(webglSceneSource)],
     ['webgl page flips require resident back textures before animation starts', /prepareStaticPageForFlip\(flip, prewarm = null\)/.test(source) && /flip-back-texture-missing/.test(source) && /return false;/.test(methodBody(source, 'prepareStaticPageForFlip')) && /flipTexturePreflight:ready/.test(source) && /if \(!prepareStaticPageForFlip\(flip, options\.prewarm \|\| null\)\) \{[\s\S]*return false;[\s\S]*\}/.test(source)],
+    ['webgl fast page flips preflight the actual target spread', /firstFlip\.targetSpread = Number\.isFinite\(Number\(options\.targetSpread\)\)/.test(source) && /if \(!prepareStaticPageForFlip\(firstFlip, options\.prewarm \|\| null\)\) return false/.test(source)],
     ['markup and 3d pagination accept full-page images', /'full'/.test(markupParserSource) && /size === 'full'/.test(bookPaginationSource)],
     ['story history can persist 3d pagination decisions', /persistPaginationMetrics/.test(bookPaginationSource) && /collectPaginationMetrics/.test(bookPaginationSource) && /pageStart/.test(storyHistorySource) && /pagination: metrics\.pagination/.test(storyHistorySource)]
 ];