Bug 315487 – CSS dotted/dashed text-decoration phase resets at bidi-run and inline-fragment boundaries

• Component: CSS
• Status: NEW

[Karl Dubost]

Steps:
Apply `text-decoration: dotted underline` to a block containing either 
* mixed-direction text (fooשלוםbaz) 
* or an inline child  (foo<span>bar</span>baz).

Expected: 
A single continuous dotted underline across the whole text run (one decorating box per css-text-decor-4 §line-decoration
(https://drafts.csswg.org/css-text-decor-4/#line-decoration)).

Actual: 
Dot phase restarts at each direction-run / inline-fragment boundary, producing visibly discontinuous dot patterns. Same defect for dashed.

Tests: 
imported/w3c/web-platform-tests/css/css-text-decor/text-decoration-dotted-001.html
imported/w3c/web-platform-tests/css/css-text-decor/text-decoration-dotted-002.html
both currently [ ImageOnlyFailure ] at  LayoutTests/TestExpectations:4539-4540.

Root cause: 
TextDecorationPainter.cpp:53 maps Dotted/Dashed to StrokeStyle::Dotted/DashedStroke; the dot-phase origin is the per-segment start point, not the decorating-box start.

[Karl Dubost]

Created attachment 479796 [details]
Diagrams of the issue

pre and post diagrams illustrating the issue

Each grey rectangle is one text-box (foo, bar, baz) painted on a single line; orange dashed lines mark text-box boundaries. The horizontal hatched strip below each row of boxes is the underline area; coloured rectangles inside it are the dot rects the helper appends. Red rects are the pre-fix output, green rects are the post-fix output. Numbers under each strip are the gaps between consecutive painted dots in pixels (red = pattern broken, green = boundary stitched cleanly). The blue vertical line and label in the post-fix row mark the phaseOrigin; the j=0..6 tick row beneath it is the world-aligned phase grid phaseOrigin + j × 30.

[Karl Dubost]

Created attachment 479797 [details]
01-inline-fragment-thickness

Testing different thickness

[Karl Dubost]

Created attachment 479798 [details]
rendering in patched minibrowser, safari, firefox, chrome

[Radar WebKit Bug Importer]

<rdar://problem/177876088>

[Karl Dubost]

Created attachment 479799 [details]
02-bidi-mixed

The current patch fails with the case fooשלוםbaz.
But maybe we can fix that too.

[Karl Dubost]

Created attachment 479800 [details]
03-dashed-style

03-dashed-style is being fixed by the current patch.

[Karl Dubost]

Created attachment 479801 [details]
rendering in patched minibrowser, safari, firefox, chrome in 03-dashed-style

fixed for 03-dashed-style

[Karl Dubost]

Created attachment 479804 [details]
04-color-matrix

Double check if it is working with different colors.

[Karl Dubost]

Created attachment 479805 [details]
rendering in patched minibrowser, safari, firefox, chrome in 04-color-matrix

Fixed with colors for 04-color-matrix

[Karl Dubost]

Created attachment 479806 [details]
05-multiple-spans

05 — three-fragment chains

Multiple inner inline boxes mean more boundaries. With period 40px and arbitrary text-box widths, every boundary has a high probability of phase mismatch pre-fix.

[Karl Dubost]

Created attachment 479807 [details]
rendering in patched minibrowser, safari, firefox, chrome in 05-multiple-spans

fixed for  05-multiple-spans

[Karl Dubost]

Created attachment 479808 [details]
06-line-through

06 — line-through continuity

Line-through goes through paintForegroundDecorations rather than paintBackgroundDecorations. The fix wires phaseOriginX into ForegroundDecorationGeometry too, so the line-through pattern is also continuous.

[Karl Dubost]

Created attachment 479809 [details]
rendering in patched minibrowser, safari, firefox, chrome in 06-line-through

Not fixed by the current patch.

[Karl Dubost]

Created attachment 479810 [details]
08-side-by-side

Left column: the failing pattern (with inner <span> or bidi). Right column: hand-shaped reference text without internal boundaries — the patched left column should visually match the right.

[Karl Dubost]

Created attachment 479811 [details]
rendering in patched minibrowser, safari, firefox, chrome in 08-side-by-side

This is not fixed by the current patched for 08-side-by-side

[Karl Dubost]

Pull request: https://github.com/WebKit/WebKit/pull/65622