For a paragraph that overflows onto the next spread, the continuation page was redrawn
synchronously after the flip (drawSpread on the main thread), so the next page stayed
blank for ~2.7s and then the carried-over lines popped in already ~24% revealed instead
of animating from the start.
Move that work off the critical path: during lookahead, prepare and cache the
continuation spread's reveal plan using the not-yet-committed preview spreads (so per-line
timing is computed across both spreads), then reuse it after the flip instead of redrawing.
- pagination: expose the preview spread layout on the returned preview spread so the owner
can detect the continuation spread (race-free; each call owns its preparedSpreads).
- renderer: revealSpreadSourceOverride lets region collection use preview spreads during
lookahead; prepareContinuationRevealPlan draws+caches the continuation plan (publishEvent
off); takeContinuationRevealPlan reuses it, re-stamped as an activate-phase publish.
- timeline: prepare the continuation plan during background (non-immediate) prepares;
revealContinuationSpread reuses it, falling back to the redraw when none was prepared.
Verified live on a spanning block: continuation now appears ~0.25s after the flip (was
~2.7s) at ve~3471 = the right line's duration, i.e. it animates from the start (no pop-in),
runs to ~full over the TTS, no fast-forward, no continuation-missing problems.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Georg