fix: don't swallow wheel events in the alt buffer when consumeWheelEvent() returns 0 - #6118
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Xukun-Liu wants to merge 1 commit into
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fix: don't swallow wheel events in the alt buffer when consumeWheelEvent() returns 0#6118Xukun-Liu wants to merge 1 commit into
Xukun-Liu wants to merge 1 commit into
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…returns 0 Wheel events in the alternate screen (no scrollback) are converted into a single up/down sequence so full-screen TUI apps can scroll. Since 5.6.0-beta.119 this path is gated on consumeWheelEvent() returning a non-zero amount, which silently drops the event when the pixel delta dampens and floors to zero lines (|deltaY| < ~50px on typical setups), when shift is held, or when renderer dims are not ready yet. The magnitude is never used by this path - the sequence sent is always exactly one ESC[A/ESC[B - so the gate is purely destructive and contradicts the comment above it. Keep the call to sync the partial scroll accumulator for the wheel-scroll option, but don't gate on it. Byte-level A/B evidence (VS Code 1.104 beta.118 vs 1.105 beta.119, instrumented bundle + raw-mode pty listener in the alt screen): 1.104 sends one ESC[B for every probe; 1.105 swallows all pixel probes below ~50px while line-mode and large deltas pass.
This was referenced Aug 20, 2026
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Summary
Since 5.6.0-beta.119 the alt-buffer wheel handler gates on
consumeWheelEvent()returning a non-zero amount, and drops the event when it rounds to zero. That kills the wheel for full-screen TUI apps on trackpads, smooth wheels and low-sensitivity scroll settings: any single pixel delta below roughly one cell height gets dampened, floored to 0, and never reaches the app. It also swallows shift+wheel entirely.The sequence this path sends is always exactly one ESC[A/ESC[B, the return value is only used as a boolean, and the comment above the code already states the behavior is "simply send a single up or down sequence". So the gate is leftover from the old line-scrolling implementation and has nothing to gain here.
This keeps the
consumeWheelEvent()call so the partial-scroll accumulator stays in sync, but onlydeltaY === 0skips sending.Fixes #6105 (resubmit of #6104, which was closed when its head repository was removed).
Verification
Byte-level A/B between VS Code 1.104 (xterm 5.6.0-beta.118) and 1.105 (5.6.0-beta.119): raw-mode pty listener in the alt screen, instrumented bundle logging
consumeWheelEvent()input and result per event.1.104 sent one ESC[B for every probe, 1.105 dropped all pixel probes under 50px (0 bytes on the pty) while line-mode and large deltas passed. Renderer dims were healthy in both runs, so the failures are the dampening path, not a sizing race.
Also re-verified on the current stable release, VS Code 1.128.0 (ships xterm 6.1.0-beta.288), same setup: pixel deltas below the threshold still produce 0 bytes on the pty, line-mode and large deltas pass. The regression is live in the release most users run today.
Tests: 4 new unit cases (small pixel delta, negative delta, zero delta, app-handles-wheel-itself). Full unit suite passes (2342 tests), lint clean.