text-decoration-inset sampler

#468928416 style + painting #498974811 percentages #539095434 overflow + repaint Status: 3-CL chain, validated locally

Feature state on this browser

Checking...Running feature detection.

If OFF: chrome://flags/#enable-experimental-web-platform-features or --enable-blink-features=CSSTextDecorationInset.

Core samples (#468928416)

Length and auto insets on a single underlined word. Each card: rendered vs expected PNG from a patched content_shell.

Percentage samples (#498974811)

Percentages resolve against the decoration width; with box-decoration-break: slice against the whole decorated run (see next section).

Decorated run + repaint (#539095434)

Run-based percentage resolution (WPT 028/030 analogues), exact ink overflow, and repaint on runtime inset changes.

repaint case: line 1 starts with inset: 30px and loses it after the first frame (must repaint to full width, no leftover trim); line 2 gains inset: -20px (must extend past the text without clipping).

state: initial

SVG samples

SVG fragments (tspans) form one decorated run, like HTML fragments.

Fragmentation and writing-mode matrix

inset: 2ch -2ch on a wrapped inline; slice trims only run edges, clone trims every line box. RTL uses bidi-override.

Debug commands

# rendering + parsing
third_party/blink/tools/run_web_tests.py -t Default --no-show-results \
  external/wpt/css/css-text-decor/ | grep text-decoration-inset

# CL3 (crbug.com/539095434): run percentages, forced breaks, invalidation
third_party/blink/tools/run_web_tests.py -t Default --no-show-results \
  external/wpt/css/css-text-decor/text-decoration-inset-028.html \
  external/wpt/css/css-text-decor/text-decoration-inset-030.html \
  external/wpt/css/css-text-decor/text-decoration-inset-percentage-slice.html \
  external/wpt/css/css-text-decor/invalidation/text-decoration-inset-invalidation.html

out/Default/blink_unittests --gtest_filter="InlinePaintContextTest.*"

# regenerate expected images (forces --enable-blink-features=CSSTextDecorationInset)
DISPLAY=:21 python3 capture_screenshots.py

Expected behavior checklist

Insets apply from logical start/end (RTL and sideways modes included).
Negative insets extend the line past the text without clipping.
auto keeps adjacent underlines distinct: clamp(thickness/2, 1px, 2px).
Percentages with slice resolve against the whole run: AB<span>CD</span> == ABCD.
Soft wrap and forced <br> keep one run: start trim on the first line, end trim on the last, percentages of the total.
Runtime inset changes repaint cleanly (no leftover trim, no clipped extension).
clone vs slice changes per-line edge behavior.