[yicheng47]

Bug

In a multi-pane chat tab, sibling panes render correct layout but wrong glyphs ("gibberish" — see below) once an agentic CLI (claude-code / codex) in another pane paints a busy frame. Text stays selectable/correct underneath; only the WebGL render is garbled. It shows up when one pane is starting/busy while a sibling is idle, and per-pane resume/start visibly disturbed siblings.

Root cause

This is an upstream xterm.js WebGL texture-atlas page-merge defect — xtermjs/xterm.js#5883 (merged by Tyriar 2026-05-21), not our code:

• Two bugs — a stale texture binding after a page swap, and stale vertex-buffer refs when a merge fires mid-update — sample glyphs from the wrong atlas page → correct layout, wrong glyphs.
• Triggered by "a much wider range of character styles" (Tyriar's words) = exactly agentic CLIs with heavy color/bold/italic/unicode, which grow the glyph atlas until it merges pages.
• Cross-platform (reproduced on Chromium as well as WKWebView/Tauri), and it predated the in-app atlas workaround — which is why the old clearTextureAtlas shotgun never fixed it and only added sibling flicker.

Fix

Bump @xterm/* to the fixed beta train (the exact set VSCode consumes — microsoft/vscode#317927):

package	before	after
@xterm/xterm	6.0.0	6.1.0-beta.220
@xterm/addon-webgl	0.19.0	0.20.0-beta.219
@xterm/addon-fit	0.11.0	0.12.0-beta.219
@xterm/addon-web-links	0.11.0	0.13.0-beta.219

Then delete the now-dead cross-pane atlas machinery (atlasPeers fan-out, refreshAllTerminalAtlases, the per-terminal repaint/rebuild hooks, and the redundant activation-path clearTextureAtlas) that only masked the symptom. WebGL is kept per-terminal, along with the onContextLoss → DOM fallback and the font-change atlas rebuild.

Also included (unrelated tidy)

refactor(ui): scope pane-group geometry sync to layout changes — ChatPaneGroup's geometry sync ran on every commit (empty deps), redoing an all-panes getBoundingClientRect + write on unrelated re-renders. Now scoped to [layout.root], with a sync when a terminal wrapper's ref mounts so restored-split hydration still positions late-attaching panes. Separate commit; safe to review independently.

Caveat

@xterm/xterm core is a beta release — the fix is not in a stable tag yet. VSCode ships this same beta, and it's a minor (6.0 → 6.1) bump, so risk is low, but flagging it since it adopts a beta dependency.

Testing

• pnpm exec tsc --noEmit, pnpm run lint, pnpm run build (tsc && vite build) — all clean.
• Working-tree diff reviewed clean by the crew reviewer (deps pinned, peer ranges line up, pnpm install --frozen-lockfile --offline clean, git diff --check clean).
• Runtime visual verification confirmed: the exact 3-pane repro no longer garbles sibling panes.

🤖 Generated with Claude Code