flushPendingWrites() captured the viewport of a user who was reading
scrollback, called terminal.write(), and restored the anchor on the next
line. xterm parses on its own schedule, so at that point the buffer has not
moved: the guard `viewportY !== preserveViewportY` was false, scrollToLine
was never called at all, and the Codex redraw landed a tick later and took
the viewport to the live bottom with nothing left to pull it back. Scrolling
up during a stream still got dragged down, which is what #358 reports, and a
refresh was the only way back to a coherent view.
The restore moves inside xterm's write callback, the first moment the
redraw's effect exists, and runs before _scheduleTerminalWriteFlush() so a
deferred remainder re-captures the restored anchor rather than the bottom.
Two things follow from it running later:
- A live anchor now wins over the sticky scroll-to-bottom. The two are
captured at different moments (_wasAtBottomBeforeWrite at the frame's
first batchTerminalWrite, the anchor at flush time), so a scroll-up in
between leaves both set, and running both would jump to the bottom and
come back a frame later instead of staying put.
- The anchor is dropped if the active session changed or a buffer load
started while the write was in flight. It indexes the buffer it was
captured from, and selectSession() resets the terminal and chunk-loads a
different scrollback.
The existing regression passed throughout, because its write mock moved the
viewport synchronously, which real xterm never does. The harness now models
an asynchronous parse (redraw lands, then the callback fires), and all five
of the anchor tests fail against the old code.
Fixes#358
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Two render-polish fixes for codex sessions in the terminal write pipeline:
- flushPendingWrites uses a 32KB first-frame budget for codex (vs 64KB for
other modes). Codex's TUI emits dense synchronized redraws during
thinking/high-effort phases; a smaller first frame keeps per-frame
xterm/WebGL stalls short and avoids multi-second main-thread blocks.
- Sticky-scroll now honours a short grace window after a manual scroll-up
gesture (USER_SCROLL_STICKY_SUPPRESS_MS = 1500ms). High-frequency codex
"Working (Ns)" status ticks were snapping the viewport back to the bottom
while the user tried to read earlier output. The wheel/touch scroll
handlers record the gesture (_noteTerminalUserScroll); flushPendingWrites
suppresses the auto-scroll-to-bottom and restores the preserved viewport
via scrollToLine while the grace window is active.
Adds test/terminal-flush-budget.test.ts (vm-sandbox harness over
terminal-ui.js): codex vs non-codex first-frame budget, buffer-load
ownership, and the scroll-up suppression / viewport restore.
Co-Authored-By: Saqeb Akhter <saqeb.akhter@gmail.com>
Codeman maintainer