Terminal: 1-frame glyph-size flicker on tab switch with CJK cell-width experiment

[im-ian]

Summary

The opt-in CJK terminal cell-width correction experiment (Settings → Experiments) currently exhibits a 1-frame glyph-size shift on tab switch when the visible pane size changes. Tracks #164 follow-up work.

Repro

1. Settings → Terminal → set Font family to a CJK monospaced font where W and 가 measure the same advance (e.g. D2Coding, Sarasa Mono K, IA Writer Mono V)
2. Settings → Experiments → enable CJK terminal cell-width correction
3. Open at least two sessions and switch between tabs while their panes differ in size

Expected: tab swap is a clean cut between rendered buffers.
Actual: 1 frame at the natural W advance is visible mid-swap — characters look briefly larger ("커졌다가") before snapping back to the halved cell width.

Root cause

Frame-by-frame inspection of a screen capture confirmed the transient. The portal target div hosting xterm moves limbo → pane body on tab activation. ResizeObserver fires → fitAddon.fit() → core._renderService.handleResize → DomRenderer._updateDimensions:

this.dimensions.device.cell.width =
  this.dimensions.device.char.width + Math.round(letterSpacing);

resets cell.width to the raw W advance (× dpr). The addon's post-fit patchTerminalCellMeasurements reapplies the halved value, but a paint can occur in between, producing the visible jump.

What we tried (all kept the transient)

• Module-level measurement memo + fast-path early return on cached + already-at-target. Avoided redundant widthCache.clear + recalibrateDefaultSpacing rounds but did not close the resize-time gap.
• _charSizeService.width getter intercept via Object.defineProperty so xterm's _updateDimensions would read the halved value from the start. Behavior unchanged in app.
• renderer.handleResize wrap (call original, then applyCellWidth(target) in the same sync tick before next paint). Behavior unchanged in app.
• term.options.letterSpacing public option (with dpr correction since xterm folds it into device.cell.width not the CSS value). Behavior unchanged in app.

Possible next steps

• Replace the xterm.js DOM renderer with a forked variant whose _updateDimensions reads the half-cell target as the canonical char width.
• Replace fitAddon with a custom fit that does not call core._renderService.clear() when only the surrounding pane size changes.
• Investigate whether a requestAnimationFrame deferral of the patch reapply could collapse into the same paint as xterm's resize repaint (instead of one extra paint after).
• Render the terminal at the actual pane dimensions while in limbo (currently fixed 800×600) so tab activation does not trigger a resize.

Workaround

The toggle ships off by default. Users hitting the original D2Coding-style misalignment can opt in and accept the flicker; the Experiments row description spells out the limitation.

References

• PR: 터미널 CJK 셀 폭 렌더링 수정 #164
• Files: src/lib/terminal-cjk-cell-width-addon.ts, src/components/Terminal.tsx, src/components/SettingsModal.tsx