Reuse spanning-aware prepared plan at activate (kill pre-playback pause)

A spanning block stalled ~2.1s before it began playing: activate ran prepareRevealBlock
with forceRebuild, synchronously redrawing the start spread (and preloading the
continuation spread) on the main thread, because the lookahead plan had been built with
right-only timing before pagination committed the overflow.

Build the start-spread plan spanning-aware during lookahead instead: when the preview
layout shows the block overflows, derive its timing across both preview spreads (via the
revealSpreadSourceOverride) and cache it. activate then reuses that plan — the same fast
cached-plan path non-spanning blocks already use — with no synchronous redraw. forceRebuild
is kept only as a fallback when a block spans but was not prepared spanning-aware (e.g. an
immediate prepare with no preview layout), and an evicted plan still rebuilds correctly
because pagination is committed by then.

Verified live: the spanning block's pre-playback gap dropped from ~2088ms to 139ms (equal
to non-spanning blocks), while the right line still reveals over its area share (~3.3s),
the continuation still animates from the start, and there are no fast-forwards or problems.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/js/book-playback-timeline-module.js

@@ -206,31 +206,40 @@ class BookPlaybackTimelineModule extends BaseModule {
         });
         if (!previewSpread) return null;
 
-        const revealDetail = this.createRevealDetail(sentence, previewSpread, 'prepare');
+        // Detect a spanning block from the (not-yet-committed) preview layout so its reveal
-        const texturePlan = this.textureRenderer.prepareRevealBlock(revealDetail, {
+        // plan can be prepared spanning-aware in the background, off the critical path. Only
-            phase: 'prepare',
+        // during background prepares and when the preview layout is available; otherwise we
-            publishEvent: false
+        // fall back to the synchronous activate rebuild + continuation redraw (today's path).
-        });
+        const previewSpreads = options.immediate !== true && Array.isArray(previewSpread.previewSpreads)
-
+            ? previewSpread.previewSpreads
-        // If this block overflows onto the next spread, prepare that continuation spread's
+            : null;
-        // reveal plan now (lookahead/background) so revealContinuationSpread reuses it after the
-        // flip instead of redrawing synchronously. Only during background prepares and when the
-        // preview layout is available; otherwise the continuation falls back to the redraw path.
-        if (options.immediate !== true && Array.isArray(previewSpread.previewSpreads)) {
         const startIndex = Math.max(0, Number(previewSpread.index || 0));
-            const continuationSpread = previewSpread.previewSpreads
+        const continuationSpread = previewSpreads
+            ? (previewSpreads
                 .filter(spread => spread
                     && Number(spread.index) > startIndex
                     && this.getBlockRevealSides(spread, sentence.blockId).length > 0)
-                .sort((a, b) => Number(a.index) - Number(b.index))[0] || null;
+                .sort((a, b) => Number(a.index) - Number(b.index))[0] || null)
+            : null;
+        const spanningPlanPrepared = Boolean(continuationSpread);
+
+        const revealDetail = this.createRevealDetail(sentence, previewSpread, 'prepare');
+        // For a spanning block, derive the start spread's timing across both preview spreads so
+        // the cached plan already spans both pages and activate can reuse it (no rebuild).
+        const texturePlan = this.textureRenderer.prepareRevealBlock(
+            spanningPlanPrepared ? { ...revealDetail, previewSpreads } : revealDetail,
+            { phase: 'prepare', publishEvent: false }
+        );
+
+        // Also prepare the continuation spread's plan so revealContinuationSpread reuses it
+        // after the flip instead of redrawing synchronously.
         if (continuationSpread) {
             this.textureRenderer.prepareContinuationRevealPlan?.({
                 ...revealDetail,
-                    previewSpreads: previewSpread.previewSpreads,
+                previewSpreads,
                 continuationSpread
             });
         }
-        }
 
         const targetSpreadIndex = Math.max(0, Number(previewSpread.index || 0));
         const currentSpreadIndex = this.getVisibleSpreadIndex();
@@ -247,6 +256,9 @@ class BookPlaybackTimelineModule extends BaseModule {
             revealSides,
             requiresPreFlip: targetSpreadIndex > currentSpreadIndex,
             requiresRightFlip: revealSides.includes('right') && this.requiresRightPageFlipAfterReveal(previewSpread),
+            // True when this block spans and its plan was prepared spanning-aware in the
+            // background, so activate can reuse it without a synchronous forceRebuild.
+            spanningPlanPrepared,
             // Snapshot the reveal timings now. A reused lookahead segment can be played by
             // a sentence instance whose animation timings were lost; without them the
             // reveal can't be word-paced and stretches across the whole TTS.
@@ -313,13 +325,14 @@ class BookPlaybackTimelineModule extends BaseModule {
         }
         const spread = segment.activeSpread || segment.previewSpread;
         const revealDetail = this.createRevealDetail(sentence, spread, 'activate');
-        // For a spanning block the prepared reveal plan was built during lookahead before
+        // A spanning block needs timing across both pages. When the plan was prepared
-        // the continuation was committed, so it is right-only. Rebuild from the committed
+        // spanning-aware during lookahead (the common case), reuse it — no synchronous redraw
-        // spreads so the reveal timing spans both pages (right page no longer absorbs the
+        // on the critical path. Only when it spans but was NOT prepared spanning-aware (e.g. an
-        // whole duration) and the continuation has reveal regions.
+        // immediate prepare with no preview layout) do we rebuild from the committed spreads.
+        const forceRebuild = segment.spansToNextSpread === true && segment.spanningPlanPrepared !== true;
         const texturePlan = this.textureRenderer.prepareRevealBlock(revealDetail, {
             publishEvent: false,
-            forceRebuild: segment.spansToNextSpread === true
+            forceRebuild
         });
         segment.activeTexturePlan = texturePlan;
         this.applyTexturePlan(texturePlan, segment, 'activate');

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

@@ -916,17 +916,30 @@ class BookTextureRendererModule extends BaseModule {
         this.revealPublishBlockIds = new Set([id]);
         const spread = detail.spread || this.currentSpread || this.pagination?.getCurrentSpread?.();
         const sides = ['left', 'right'];
-        const published = this.drawSpread(spread, sides, {
+        // When the caller supplies the (not-yet-committed) preview spreads for a spanning
+        // block, derive this spread's reveal timing across all of them so the cached plan
+        // already spans both pages. activate can then reuse it instead of forcing a
+        // synchronous rebuild on the critical path.
+        const spanningPreview = Array.isArray(detail.previewSpreads) && detail.previewSpreads.length > 1;
+        const previousOverride = this.revealSpreadSourceOverride;
+        if (spanningPreview) this.revealSpreadSourceOverride = detail.previewSpreads;
+        let published = null;
+        try {
+            published = this.drawSpread(spread, sides, {
                 phase,
                 publishEvent: options.publishEvent !== false
             });
-        this.preloadAdditionalRevealSpreads(id, spread);
+        } finally {
+            this.revealSpreadSourceOverride = previousOverride;
+        }
+        if (!spanningPreview) this.preloadAdditionalRevealSpreads(id, spread);
         if (phase === 'prepare' && published) {
             this.pageCache?.rememberPreparedRevealPlan?.(id, {
                 ...published,
                 blockId,
                 wordTimings,
-                totalDuration: detail.totalDuration || 0
+                totalDuration: detail.totalDuration || 0,
+                spanningTimingPrepared: spanningPreview === true
             });
         }
         this.markPipelineTiming('prepareRevealBlock:end', {

scripts/check-webgl-book-lab.js

@@ -186,7 +186,7 @@ const checks = [
     ['webgl flip assigns explicit source and back page textures before animation starts', /resolveCurrentFlipSourceTexture\(sourceSide\)/.test(source) && /const targetBackSide = flip\.direction > 0 \? 'left' : 'right'/.test(source) && /const targetBackPageMeta = getPaginationPageMeta\(targetBackPageIndex\) \|\| makeBlankPageMeta\(targetBackPageIndex\)/.test(source) && /materials\.flipPageSurface\.map = sourceTexture/.test(source) && /materials\.flipPageBackSurface\.map = backDeferred \? getBlankPageTexture\(\) : \(backTexture \|\| getBlankPageTexture\(\)\)/.test(source)],
     ['webgl flip never falls back to the opposite visible stack for target back texture', /function resolveFlipBackTexture\(pageMeta = null, prewarmedTexture = null\)/.test(source) && source.includes('return pageTextureStore?.getResidentTextureForMeta?.(pageMeta);') && !/oppositeMaterial\?\.map/.test(methodBody(source, 'prepareStaticPageForFlip'))],
     ['webgl page texture record metadata normalizes omitted or null sides into explicit blank pages', /function normalizePageMetaPair/.test(source) && /function makeBlankPageMeta/.test(source) && /applyExplicitBlankPageTexture/.test(source) && /normalizePageTextureRecordDetail/.test(source) && !/hasLeftMeta/.test(methodBody(source, 'handlePageTextureRecords'))],
-    ['texture renderer publishes both spread sides for reveal preparation', /const sides = \['left', 'right'\]/.test(textureRendererSource) && /const published = this\.drawSpread\(spread, sides/.test(textureRendererSource) && /pageMeta: prepared\.pageMeta \|\| \{\}/.test(textureRendererSource)],
+    ['texture renderer publishes both spread sides for reveal preparation', /const sides = \['left', 'right'\]/.test(textureRendererSource) && /published = this\.drawSpread\(spread, sides/.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)],
     ['texture store queues newer same-page cache writes instead of dropping them', /storePageCanvas/.test(webglPageCacheSource) && /isOlderPageMeta/.test(webglPageCacheSource) && /const previousWrite = pending\?\.promise \|\| Promise\.resolve\(\)/.test(webglPageCacheSource) && /pendingPageWrites\.set\(key, \{[\s\S]*pageMeta: \{ \.\.\.\(pageMeta \|\| \{\}\) \}/.test(webglPageCacheSource)],
     ['webgl texture store resident cache reuses newest page version for older readiness requests', /isOlderPageMeta/.test(webglPageCacheSource) && /getResidentTextureForMeta/.test(webglPageCacheSource) && /if \(!resident\) return null/.test(webglPageCacheSource) && !/if \(!resident \|\| this\.isOlderPageMeta\(pageMeta, resident\.pageMeta\)\) return null/.test(webglPageCacheSource)],
@@ -226,8 +226,9 @@ const checks = [
     ['texture renderer derives reveal regions from the just-drawn preview spread before committed pagination', /if \(this\.currentSpread\) sourceSpreads\.push\(this\.currentSpread\)/.test(textureRendererSource) && /paginationSpreads\.forEach/.test(textureRendererSource) && /Number\(spread\.index\) === Number\(this\.currentSpread\.index\)/.test(textureRendererSource)],
     ['texture renderer prepares a spanning block continuation spread in the background and reuses it (no synchronous redraw on the critical path)', /revealSpreadSourceOverride/.test(textureRendererSource) && /prepareContinuationRevealPlan/.test(textureRendererSource) && /takeContinuationRevealPlan/.test(textureRendererSource) && /`\$\{id\}:cont`/.test(textureRendererSource) && /prepareContinuationRevealPlan/.test(bookPlaybackTimelineSource) && /takeContinuationRevealPlan/.test(bookPlaybackTimelineSource) && /previewSpreads/.test(bookPaginationSource)],
     ['texture renderer preloads every spread touched by an active reveal block', /preloadAdditionalRevealSpreads/.test(textureRendererSource) && /spreadContainsBlock/.test(textureRendererSource) && /this\.drawSpread\(spread, \['left', 'right'\], \{ phase: 'prepare' \}\)/.test(textureRendererSource)],
+    ['book playback timeline reuses the spanning-aware prepared plan at activate instead of a synchronous forceRebuild', /spanningPlanPrepared/.test(bookPlaybackTimelineSource) && /forceRebuild = segment\.spansToNextSpread === true && segment\.spanningPlanPrepared !== true/.test(bookPlaybackTimelineSource) && /spanningTimingPrepared/.test(textureRendererSource) && /this\.revealSpreadSourceOverride = detail\.previewSpreads/.test(textureRendererSource)],
     ['webgl visible spread is owned by scene flips, not pagination publishes', /spreadUpdate:state-only/.test(source) && /webglBookPlaybackActive/.test(source) && /spreadUpdate:jump/.test(source) && /window\.BookTextureRenderer\?\.drawSpread\?\.\(spread, \['left', 'right'\], \{ force: true \}\)/.test(source)],
-    ['3D overflow reveal preloads target spread before forced page flip', /createRevealDetail\(sentence, previewSpread, 'prepare'\)/.test(bookPlaybackTimelineSource) && /this\.textureRenderer\.prepareRevealBlock\(revealDetail, \{[\s\S]*phase: 'prepare'[\s\S]*publishEvent: false/.test(bookPlaybackTimelineSource) && /this\.prewarmSegmentTextures\(segment\)/.test(bookPlaybackTimelineSource) && /this\.assertSegmentReady\(segment, 'prepare'\)/.test(bookPlaybackTimelineSource) && /reason: 'timeline-preplay-spread-transition'/.test(bookPlaybackTimelineSource)],
+    ['3D overflow reveal preloads target spread before forced page flip', /createRevealDetail\(sentence, previewSpread, 'prepare'\)/.test(bookPlaybackTimelineSource) && /this\.textureRenderer\.prepareRevealBlock\(\s*[\s\S]*revealDetail[\s\S]*phase: 'prepare'[\s\S]*publishEvent: false/.test(bookPlaybackTimelineSource) && /this\.prewarmSegmentTextures\(segment\)/.test(bookPlaybackTimelineSource) && /this\.assertSegmentReady\(segment, 'prepare'\)/.test(bookPlaybackTimelineSource) && /reason: 'timeline-preplay-spread-transition'/.test(bookPlaybackTimelineSource)],
     ['book playback timeline is loaded through module infrastructure', /book-playback-timeline-module\.js/.test(loaderSource) && /super\('book-playback-timeline'/.test(bookPlaybackTimelineSource) && /reportProgress\(100, 'Book playback timeline ready'\)/.test(bookPlaybackTimelineSource)],
     ['book playback timeline initializes before sentence queue without a dependency cycle', /this\.dependencies = \[[^\]]*'book-playback-timeline'[^\]]*\]/.test(sentenceQueueSource) && !/this\.dependencies = \[[^\]]*'sentence-queue'[^\]]*\]/.test(bookPlaybackTimelineSource) && /calculateAnimationTiming\(words = \[\]/.test(bookPlaybackTimelineSource)],
     ['3D display playback is owned by book playback timeline', /book-playback-timeline/.test(uiDisplayHandlerSource) && /playWebGLBookSentence/.test(uiDisplayHandlerSource) && /timeline\.playSentence\(sentence\)/.test(uiDisplayHandlerSource) && /if \(useWebGLBookReveal\) \{[\s\S]*await this\.playWebGLBookSentence\(sentence\);[\s\S]*this\.markBlockRendered\(sentence\.blockId\);[\s\S]*return null;/.test(uiDisplayHandlerSource)],