fix: restore cached prefix fits

Author: chenglou

src/layout.test.ts

@@ -897,6 +897,15 @@ describe('layout invariants', () => {
     expect(reconstructFromLineBoundaries(prepared, result.lines)).toBe('foo trans\u00ADatlantic')
   })
 
+  test('soft-hyphen fallback does not crash when overflow happens on a later space', () => {
+    const prepared = prepareWithSegments('foo trans\u00ADatlantic labels', FONT)
+    const width = measureWidth('foo transatlantic', FONT) + 0.1
+    const result = layoutWithLines(prepared, width, LINE_HEIGHT)
+
+    expect(result.lines.map(line => line.text)).toEqual(['foo transatlantic ', 'labels'])
+    expect(layout(prepared, width, LINE_HEIGHT).lineCount).toBe(result.lineCount)
+  })
+
   test('layoutNextLine variable-width streaming stays contiguous and reconstructs normalized text', () => {
     const prepared = prepareWithSegments(
       'foo trans\u00ADatlantic said "hello" to 世界 and waved. According to محمد الأحمد, alpha\u200Bbeta 🚀',

src/line-break.ts

@@ -466,7 +466,8 @@ export function walkPreparedLines(
 
   function continueSoftHyphenBreakableSegment(segmentIndex: number): boolean {
     if (pendingBreakKind !== 'soft-hyphen') return false
-    const fitWidths = breakableFitAdvances[segmentIndex]!
+    const fitWidths = breakableFitAdvances[segmentIndex]
+    if (fitWidths == null) return false
     const { fitCount, fittedWidth } = fitSoftHyphenBreak(
       fitWidths,
       lineW,

src/measurement.ts

@@ -227,18 +227,15 @@ export function getSegmentBreakableFitAdvances(
     return metrics.breakableFitAdvances
   }
 
-  const ctx = getMeasureContext()
   const advances: number[] = []
   const graphemeSegmenter = getSharedGraphemeSegmenter()
   let prefix = ''
   let prefixWidth = 0
-  let prefixEmojiCount = 0
 
   for (const gs of graphemeSegmenter.segment(seg)) {
     prefix += gs.segment
-    if (emojiCorrection !== 0 && isEmojiGrapheme(gs.segment)) prefixEmojiCount++
-
-    const nextPrefixWidth = ctx.measureText(prefix).width - prefixEmojiCount * emojiCorrection
+    const prefixMetrics = getSegmentMetrics(prefix, cache)
+    const nextPrefixWidth = getCorrectedSegmentWidth(prefix, prefixMetrics, emojiCorrection)
     advances.push(nextPrefixWidth - prefixWidth)
     prefixWidth = nextPrefixWidth
   }