Bug 291495 – Misses word-break opportunity and overflows

• Component: Layout and Rendering
• Status: RESOLVED FIXED

[Johannes Odland]

There's a small bug in how WebKit breaks overflowing content with inline boxes. 

The bug occurs when it can fit the content but not the end side padding and border of an inline box inside the available width.

Expected behavior:
It should break at an earlier point if possible, moving the remaining content, padding and border to a new line. 

Current behavior:
WebKit breaks the line at a later point, making the border and padding overflowing its container.

Reduced test case:
https://johannesodland.github.io/browserbugs/2025/safari-word-break-before-inline-box-end.html
processOverflowingContentWithText

[Ahmad Saleem]

Firefox Nightly 139 (20250413095036) and Chrome Canary 137.0.7123.0 (Official Build) canary (arm64) matches each other and split in two lines.

While Safari 18.4 and WebKit ToT (293648@main) remain in one line.

[Johannes Odland]

Here's my current understanding of the issue: 

In `InlineContentBreaker::processOverflowingContentWithText()` webkit notices that the InlineBoxEnd item will cause overflow, and stores the index of the inline box end as `overflowingRunIndex`.

Later, it tries to run `InlineContentBreaker::tryBreakingPreviousNonOverflowingRuns()`.
There it will try to break the previous text run, but the entire run fits. 
It then includes the trailing InlineBoxEnd, which overflows.

[Johannes Odland]

Maybe the width of any trailing InlineBoxEnds should be subtracted from the available space before trying to break preceding text runs?

[Radar WebKit Bug Importer]

<rdar://problem/149651067>

[alan]

Created attachment 475049 [details]
[fast-cq]Patch

[EWS]

Committed 294132@main (7953222aee54): <https://commits.webkit.org/294132@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 475049 [details].