CJK fallback glyphs are upscaled to fill double-width cells and float above the latin baseline

[AprilNEA]

Environment

restty 0.1.35 and 0.2.0 (verified on both), Electron 42.4.1 renderer, macOS (Apple Silicon), WebGPU backend. Primary font: IBM Plex Mono (buffer source), CJK fallback via local families.

Symptom

On mixed CJK/latin lines, CJK glyphs from a fallback font render noticeably larger (~1.2x) than the primary font's em size and sit visibly above the latin baseline — latin words appear to sink. Pure-latin lines are fine. Reproduced with Hiragino Sans GB and Heiti SC as the CJK fallback; end users report it as 'baseline is broken'.

ls 列出文件 file.txt 大小 1024 字节 mixed 混排 CJK   ← CJK floats high, latin sinks

Analysis (from reading the 0.1.35 bundle; the same behavior reproduces on 0.2.0)

• For fallback entries with fontMaxCellSpan > 1, the scale resolution applies a width-fit: widthAdjust = (cellW * maxSpan) / widthPx (clamped 0.5–2). With a primary mono whose advance is 600/1000 em (IBM Plex Mono, and most monos), two cells = 1.2em, so any CJK face with the standard 1000/1000 advance gets upscaled x1.2 above its em size.
• The baseline anchor (fallbackMetricAnchorUnits / ascender diff) then places the enlarged glyph high relative to the primary baseline.
• fontScaleOverrides cannot counteract it: it is only applied in the symbol-font branch of the scale resolution.
• Arial Unicode MS happens to escape because its tall line metrics trigger the adjustedHeightPx > lineHeight clamp, which shrinks it back — that's the only stock macOS font we found that keeps mixed lines on one baseline.

Suggestion

Match Ghostty's behavior for wide fallback glyphs: render at the primary em size (no width-fit upscale) and center horizontally within the double-width cell, keeping the baseline anchored. That is exactly what the accidental Arial Unicode path produces, and it looks correct.

Happy to provide the standalone Electron harness + screenshots (Hiragino GB vs Arial Unicode side by side) if useful.

[wiedymi]

Fixed and published in restty@0.2.1 via #26.

Wide fallback faces now keep their natural em scale: the double-cell width adjustment may shrink an oversized glyph, but it can no longer upscale a normal CJK em to 1.2x. Single wide fallback glyphs are also centered across their occupied cells in both WebGPU and WebGL2 while retaining the existing baseline anchor.

Regression tests cover the IBM Plex Mono-style 0.6em primary advance and double-cell centering.

Release: https://github.com/wiedymi/restty/releases/tag/v0.2.1

[wiedymi]

Follow-up regression fix published in restty@0.2.2 via #28.

After 0.2.1 stopped wide fallback upscaling, the old metric-anchor calculation could still add a vertical offset after the fallback was reduced by the line-height clamp. With the bundled Fira Code + Noto CJK metrics it added +1.1475px, matching the reported downward shift.

Ordinary text fallbacks now use the same baseline as the primary font in both WebGPU and WebGL2. Symbol and emoji-specific alignment remains separate.

Release: https://github.com/wiedymi/restty/releases/tag/v0.2.2

[wiedymi]

Second follow-up published in restty@0.2.3 via #29.

The baseline was correct in 0.2.2, but wide CJK faces were still sized from their own taller line metrics. With JetBrains Mono + Noto CJK at 18px, that reduced the ideographic advance from the primary-em 13.6364px to 12.4309px inside a 16px two-cell span, creating excessive visual gaps.

Wide text fallbacks now use the primary em, consult the actual ideographic advance for two-cell fit, and only shrink when necessary. WebGPU and WebGL2 share the same resolver.

Release: https://github.com/wiedymi/restty/releases/tag/v0.2.3

[wiedymi]

Final spacing follow-up published in restty@0.2.4 via #31.

The 0.2.3 primary-em fix was still incomplete: it bypassed Ghostty's default ic_width size adjustment for wide fallback faces. Ghostty normalizes the primary ideograph-width target and the fallback 水 advance by each face's em, then scales the fallback by their ratio.

For the bundled JetBrains Mono + Noto Sans CJK SC faces, the primary has no 水, so its target is estimated from min(printable ASCII height, two cell widths) = 1050 font units. Noto's valid 水 advance is 1000 units, producing a 1.05 adjustment. At the default 18px height, the CJK advance is now 14.3182px instead of 13.6364px inside the 16px two-cell span. The shared baseline and two-cell safety cap remain unchanged in WebGPU and WebGL2.

Regression tests cover the bundled metrics and Ghostty's invalid-ic_width fallback behavior. Please retest the original mixed Latin/CJK line with 0.2.4.

Release: https://github.com/wiedymi/restty/releases/tag/v0.2.4