[nicoburns]

Turns on Stylo's layout.css.background-clip.text.enabled pref and paints text-clipped backgrounds.

Each run of adjacent text-clipped background layers (including the background colour when the last layer is text-clipped) is painted into an isolation layer clipped to the border box. The run is then cut down with a Compose::DestIn layer containing a text mask. This is the same technique the CSS mask code uses.

The mask comes from a new dedicated draw_text_mask traversal. It walks layout_children rather than paint_children, because in-flow stacking contexts with a z-index get hoisted out of paint_children. It skips out-of-flow, display: none and non-visible nodes, and paints glyphs and text decorations with an opaque brush (a new brush_override on stroke_text). It applies transforms, scroll offsets and overflow clips, but not opacity, filters or blend modes. The transform and clip logic is now shared with render_element through node_transform, scrolled_transform and should_clip_node.

WPT css/css-backgrounds/background-clip: 13 → 25 passing. css/css-backgrounds + css/css-masking: 805 → 816 passing.

One new failure: css/css-backgrounds/background-multiple-layers-table-cell.html. Once the pref is on, its text-clipped table-row-group layer becomes valid, but table track backgrounds only paint solid colours. Still failing, due to features out of scope here:

• background-clip on inline boxes (clip-text-inline, which draw_inline_backgrounds doesn't handle)
• table track gradients
• fragmentation
• blend modes

WPT results

Subtests: 15 newly passing, 1 newly failing (net +14).

Subtest changes by area (5 areas)

+ css                 | +14 (+15 / -1) | 59.63% -> 59.64% ( +0.01%) | 118826 -> 118840 / 199259
+   css-backgrounds   | +13 (+14 / -1) | 73.32% -> 73.51% ( +0.19%) |   5117 ->   5130 /   6979
+     background-clip | +12 (+12 / -0) | 26.00% -> 50.00% (+24.00%) |     13 ->     25 /     50
+     parsing         |  +2 ( +2 / -0) | 90.35% -> 90.57% ( +0.21%) |    843 ->    845 /    933
+   css-transitions   |  +1 ( +1 / -0) | 65.86% -> 65.93% ( +0.07%) |    924 ->    925 /   1403

Full diff (16 changed tests)

+ FAIL => PASS    [1/1]  +1  css/css-backgrounds/background-clip/clip-text-descendants.html
+ FAIL => PASS    [1/1]  +1  css/css-backgrounds/background-clip/clip-text-ellipsis.html
+ FAIL => PASS    [1/1]  +1  css/css-backgrounds/background-clip/clip-text-flex.html
+ FAIL => PASS    [1/1]  +1  css/css-backgrounds/background-clip/clip-text-inline-block-child.html
+ FAIL => PASS    [1/1]  +1  css/css-backgrounds/background-clip/clip-text-multi-line.html
+ FAIL => PASS    [1/1]  +1  css/css-backgrounds/background-clip/clip-text-on-body-not-propagated-to-root.html
+ FAIL => PASS    [1/1]  +1  css/css-backgrounds/background-clip/clip-text-out-of-flow-child.html
+ FAIL => PASS    [1/1]  +1  css/css-backgrounds/background-clip/clip-text-relative-child.html
+ FAIL => PASS    [1/1]  +1  css/css-backgrounds/background-clip/clip-text-stacking-context-child-002.html
+ FAIL => PASS    [1/1]  +1  css/css-backgrounds/background-clip/clip-text-stacking-context-child.html
+ FAIL => PASS    [1/1]  +1  css/css-backgrounds/background-clip/clip-text-text-decorations.html
+ FAIL => PASS    [1/1]  +1  css/css-backgrounds/background-clip/clip-text-text-emphasis.html
- PASS => FAIL    [0/1]  -1  css/css-backgrounds/background-multiple-layers-table-cell.html
+ FAIL => FAIL   [7/11]  +1  css/css-backgrounds/parsing/background-clip-computed.html
+ FAIL => FAIL    [4/9]  +1  css/css-backgrounds/parsing/background-clip-valid.html
+ FAIL => FAIL  [31/32]  +1  css/css-transitions/all-interpolates-same-as-explicit-property.html

_{Generated by the WPT workflow.}