Font format baseline and emoji renderer contract¶

These are selected-font capability tests, not a guarantee of default font availability: they establish that a named face can be rendered, not that a suitable face will be found. Pixel class mono alone cannot distinguish a recognizable glyph from deterministic tofu, which is why artwork acceptance lives in the emoji artwork gate and candidate selection in tests/xvfb-font-discovery.sh. The 2026-09-19 fallback review records the text-presentation failures that this matrix once passed over; those are repaired, and the review is kept as the diagnosis rather than as a current defect list.

The original baseline commit captured Revenant's pre-routing Xft behavior. The same executable contract now separates two questions that otherwise look identical on screen:

1. What happens when a fixture is selected directly as faceName?
2. Does faceNameDoublesize route a wide character to that fixture while preserving the terminal engine's committed width?

The executable contract is tests/xvfb-font-baseline.sh. It runs under Xvfb with isolated Fontconfig universes, a black background, and white foreground. For each wide probe it asserts a cursor-position report at row 1, column 3, classifies every pixel in the committed two-cell rectangle, and proves the third cell contains no ink.

Current direct format behavior¶

These results use Xft 2.3.8, Cairo 1.18, and the fonts pinned by tools/font-fixtures/manifest.json:

Fixture path	U+1F600 result
CBDT at 109 ppem	color
Historical Noto 2.034 CBDT at 109 ppem	color
CBDT at 61 ppem	color
COLRv0 + genuine outline base	color
COLRv1 + empty outline base	color
Monochrome outline	foreground monochrome
SVGinOT + genuine outline base	color
sbix at 16 ppem + empty outline base	color

Noto Sans Mono CJK JP renders U+65E5 in foreground monochrome with the same two-cell geometry assertion.

Cairo 1.18 renders the selected color-glyph formats while Xft continues to render monochrome outlines. The format matrix preserves the already-working CBDT, COLRv0, and sbix results and deliberately breaks the pre-feature baseline's passing blank COLRv1 and SVG cells.

The Cairo delegate is persistent rather than constructed per cell. Its scaled-font cache centres each selected face from its own ascent and descent, and its draw call receives the effective intersection of the cell, damage, and cursor clips. The routing test guards partial-damage restoration and a width-one COLRv1 glyph under a block cursor against neighbouring-cell paint.

The ink taxonomy requires nonzero outline area, not just a glyf entry or contour bounds. OpenMoji's mapped U+1F600 base has a two-point degenerate contour and therefore records color, not outline+color; TwitterColorEmoji has a genuine outline and records outline+svg.

faceNameDoublesize boundary¶

Holding faceName at DejaVu Sans Mono while varying faceNameDoublesize exercises every color technology, monochrome outlines, and CJK CFF outlines. All fixtures now render through the role, remain inside two cells, report the cursor at column 3, and leave the third cell blank. Fixed strikes are fitted in both directions: CBDT 61/109 ppem downscale and sbix 16 ppem upscale.

tests/xvfb-emoji-routing.sh adds Unicode defaults, VS15/VS16, policy overrides, emoji-to-doublesize fall-through, CJK isolation, one-cell width preservation, genuine outline fallback, and empty/degenerate-base rejection. It also requires single-run shaping for keycap, skin-tone, ZWJ, family, regional-indicator, subdivision-tag, adjacent-flag, and combining sequences, plus atomic whole-grapheme role fallback. Current Noto paints the tested family sequence through COLRv1 with an achromatic gray palette, so the pixel probe correctly reports mono even though routing and rendering still use the color font path.

The historical Noto Emoji 2.034 fixture is byte-identical to the font embedded by Ghostty tags 1.2.1, 1.3.0, and 1.3.1. It preserves the earlier colorful family artwork from before Google's Emoji 15.1 redesign and, because it predates U+1FAE8, supplies a realistic color-face miss for the emoji fall-through test. It complements rather than replaces the current pinned Noto CBDT fixture.

The combining case then redraws ordinary primary-face text and requires normal vertical cell margins. This guards the ownership boundary between HarfBuzz and Xft: shaping uses an independent OpenType face and must not change Xft's live FreeType face state. Its policy adversaries put a color emoji font in the doublesize role under emojiPresentation: text, and put an outline-less CBDT font in the primary role under colorGlyphs: false; neither case may leak color.

General shaping and fallback¶

tests/xvfb-text-shaping.sh extends the isolated fixture contract beyond emoji. DejaVu Sans Mono supplies regular, bold, oblique, and bold-oblique faces; Noto Sans Devanagari and Noto Sans Mono CJK JP are available only through fontconfig fallback. The test requires positioned Vietnamese and Zalgo runs, a real oblique SGR-italic face, a two-backend-grapheme Devanagari conjunct shaped into one glyph across its unchanged two-cell span, and a mixed CJK/combining-text fallback run. A partial Expose beginning in the conjunct's second cell must reconstruct the same complete shaped run.

Unicode version interlock¶

The fixture routing input and selected libghostty uucode width input both declare Unicode 17.0. tools/generate-emoji-table --check rejects a mismatch between either input, the fixture manifest, and the generated C table. font-fixture-info.py --check independently checks maintained fixture data.