fix(layout-engine): render underlined tabs as one continuous line#3627
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…lay (SD-3330) Underlined tabs drew a border-bottom while adjacent text used native text-decoration, meeting at slightly different y and leaving a seam. One line-level overlay now owns the underline for both the inline and segment-positioned branches; native painters are suppressed where it covers the run. Scoped to single/double/dotted/dashed (words/wave/heavy and theme colors stay on the existing path). Tab paint-version now keys on fontSize/fontFamily/color/font epoch so a style change repaints. Covered by painter unit tests (inline + segment-positioned merged geometry), version-signature tests, overlay-scope guards, and a behavior test that types text+tabs, selects all, underlines, and asserts one continuous overlay.
…SD-3330) A tab-only paragraph has no text run, so the line fell back to synthetic 0.8/0.2 ascent/descent. Derive metrics from the tab's own font so a tab-only underlined line gets the same measured ascent/descent (underline offset, line height) as the equivalent text line. Effective only in a real browser; jsdom has no real font metrics, so validate via the layout/visual suites in CI.
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tupizz
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Jun 3, 2026
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RTL paragraphs skip segment positioning and fall to inline flow so the browser places the bidi tabs, but the underline overlay built LTR left-offsets from the line start and suppressed the native underlines - painting on the wrong side. Gate the overlay on !isRtl so RTL keeps native text-decoration + tab borders; a mirrored RTL overlay is larger work and unnecessary here. Adds a painter regression test (RTL underlined-tab line -> no overlay, natives intact).
…nes (SD-3330) The inline-flow overlay builds left-origin offsets, so it mispainted - and suppressed the natively-correct underlines - on center/right-aligned, hanging/negative-indent, and atomic-run lines. Gate it to left/start-aligned LTR lines with a matching indent origin, built only from text/tab/line-break runs; the segment-positioned branch stays exempt (it captures spans at the same x it positions runs at). Add the tab's bold/italic to the version key (tab-only metrics depend on them via getFontInfoFromRun) and pin the overlay-top unit assertions to underlineOffsetFromLineTop. Tests: center/right alignment, hanging indent, and a boolean-underline field annotation.
… off (SD-3330) Sharpen the atomic-run regression: beyond checking no overlay is produced, assert the field annotation's own underline is not suppressed (textDecorationLine not forced to 'none'), via the aria-label selector.
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Follow-up to #3611.
Draws one shared underline across text, spaces, and underlined tabs so the inline case and the tab-stop case stay continuous. Also expands the tab paint cache key and tightens tab-only line metrics to use the tab font.
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