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bug(terminal): WebGL atlas changes leave stale glyphs after scrolling #789
Description
Expected behavior
Scrolling raw terminal history and displaying colored CLI output must repaint the visible cells correctly without disturbing running sessions.
Actual behavior and evidence
User reports left-edge stale colored cells/text fragments after scrolling a plain terminal running interactive Claude on macOS after the WebGL performance change (#783). The exact screenshot cause is not yet confirmed.
An independent Electron reproduction using the installed @xterm/xterm 6.0.0 and @xterm/addon-webgl 0.19.0 found a concrete related defect: atlas page merges during colored output leave stale rendered pixels; repainting without changing terminal buffer content corrects them. Scrollback navigation also reproduced this instability. Synthetic two-terminal stress: 148 removed atlas pages, 33 output-frame mismatches and one scroll-frame mismatch, versus a subsequent explicit refresh of identical buffer contents.
Investigation and acceptance
- Verify whether scheduling a coalesced repaint after atlas layout changes resolves the deterministic reproduction.
- Preserve GPU acceleration, input/PTY ordering, terminal history, and DOM context-loss fallback.
- Cover teardown and late-import lifecycle, real rendered-pixel stability, and all shared-helper terminal hosts.
- Do not restart or kill live agents to investigate.
Related upstream investigation: xtermjs/xterm.js#6038 . Our pinned 0.19.0 implementation predates newer merge-retry logic; do not assume every upstream failure mode applies unchanged.
Activity
Juliusolsson05 commentedon Sep 5, 2026
Implementation and verification are in #790. The installed 0.19.0 addon has two reproduced mechanisms: stale cell texture coordinates after a mid-frame atlas merge, and per-page texture-version collisions that can leave an old GPU texture bound. A refresh-only experiment removed frame instability but failed an independent fresh-atlas pixel oracle, so it was not shipped. The final repair invalidates bindings and coalesces a viewport refresh on atlas layout events without clearing the shared glyph cache. Final real Electron run: 148 distinct page removals; control has 33 output-frame mismatches, one scroll mismatch, and 1,264,868 differing pixel channels against an independent reference; production loader has zero mismatches and zero reference differences. Exact screenshot confirmation remains pending deployment; no live session was restarted.
fix(terminal): stop attaching the WebGL renderer until a stable addon…