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[shaping] fix crash when wrapping part of string including forced break #266

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[shaping] fix crash when wrapping part of string including forced break #266

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[shaping] fix crash when wrapping part of string inclunding forced break
benoitkugler Jul 9, 2026
a139ad8
cleanup
benoitkugler Jul 9, 2026
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5 changes: 4 additions & 1 deletion shaping/wrapping.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1218,7 +1218,10 @@ func (l *LineWrapper) processBreakOption(option breakOption, config lineConfig)
// Fill candidate line with runs until the run containing the break option.
l.fillUntil(l.glyphRuns, option)

currRunIndex, run, _ := l.glyphRuns.Peek()
currRunIndex, run, isValid := l.glyphRuns.Peek()
if !isValid {
return breakInvalid, Output{}
}
l.mapper.mapRun(currRunIndex, run)
if !option.isValid(l.mapper.mapping, run) {
// Reject invalid line break candidate and acquire a new one.
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13 changes: 13 additions & 0 deletions shaping/wrapping_test.go
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Expand Up @@ -3623,3 +3623,16 @@ func TestWrapping_oneLine_overflow_bug(t *testing.T) {
_, done := l.WrapNextLine(maxWidth)
tu.Assert(t, done)
}

func TestWrapForcedBreak(t *testing.T) {
f := loadOpentypeFont(t, "../font/testdata/Roboto-Regular.ttf")

s := []rune("line1\nline2")

@whereswaldon whereswaldon Jul 13, 2026

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It's certainly better not to crash in this case, but isn't this API misuse? The whole shaping infrastructure expects you to perform segmentation before you shape. Segmentation should convert this input string into two runs.

Or is the problem that we previously would fail to convert this into two runs, and would shape/wrap it as a single run regardless?

I don't think I understand the full context of the problem. 😅 Sorry! I've reread the PR description a few times, but I think I'm still missing something.

@benoitkugler benoitkugler Jul 15, 2026 •
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Actually, I think I've mixed up line break and paragraph break. That's probably why the context is not very clear here.
We will probably need to tweak our bidi implementation, but I need to double check that.
Sorry for the noise...

// bidi segmentation will split the input at the first line
input := Input{Text: s, RunEnd: 5, Direction: di.DirectionLTR, Size: fixed.I(10), Face: f}
run := (&HarfbuzzShaper{}).Shape(input)

var l LineWrapper
l.Prepare(WrapConfig{BreakPolicy: Never}, []rune(s), NewSliceIterator([]Output{run}))
_, _ = l.WrapNextLine(math.MaxInt) // assert there is no crash
}
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