docs: document breakableFitAdvances cache invariant#139
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docs: document breakableFitAdvances cache invariant#139shaun0927 wants to merge 1 commit intochenglou:mainfrom
shaun0927 wants to merge 1 commit intochenglou:mainfrom
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The cache check in getSegmentBreakableFitAdvances returns the stored result without consulting the mode parameter. This is safe because the current call sites guarantee mode is determined by isNumericRunSegment (text-deterministic) and the session-invariant engine profile, so the same segment text always arrives with the same mode. Add a comment explaining this invariant so future callers know the constraint. No behavior change.
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Log pretext-upstream-rhythm session in WORKFLOW (merge f201433, checks, triage wave chenglou#139/chenglou#132, fork scan). Made-with: Cursor
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Fixes #137
Problem
getSegmentBreakableFitAdvances()accepts amode: BreakableFitModeparameter but the cache at line 203 returns the stored result without checking whether the mode matches. If a future caller requested the same segment text with a different mode, the cache would silently return the wrong advances.Why the cache is safe today
The mode is fully determined by two things that are constant for a given segment text within a session:
isNumericRunSegment(text)— pure text predicate, always returns the same result for the same string → forcespair-contextengineProfile.preferPrefixWidthsForBreakableRuns— detected once fromnavigator.userAgent, invariant for the session → forcessegment-prefixessum-graphemesSo the same segment text always arrives with the same mode, and the mode-agnostic cache never returns a mismatched result.
Change
Added a 5-line comment above the cache check explaining the invariant and the consequence of breaking it. No behavior change, no public API change. All 84 tests pass.
Test plan
bun test src/layout.test.ts— 84 pass, 0 fail