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fix(ui): coalesce viewport listener via microtask to avoid setState loop #242

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fix(ui): coalesce viewport listener via microtask to avoid setState loop #242

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27 changes: 24 additions & 3 deletions src/ui/components/panes/DiffPane.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -324,7 +324,10 @@ export function DiffPane({
return;
}

const updateViewport = () => {
let cancelled = false;
let scheduled = false;

const readViewport = () => {
const nextTop = scrollBox.scrollTop ?? 0;
const nextHeight = scrollBox.viewport.height ?? 0;

Expand All @@ -341,16 +344,34 @@ export function DiffPane({
);
};

// OpenTUI emits `change` synchronously from inside its own slider sync, and other
// useLayoutEffects in this pane scroll the box from inside React's commit phase.
// Calling setScrollViewport directly from the listener can run setState while React
// is already committing — which downstream layout effects can amplify into a render
// loop and trip React's max-update-depth guard. Coalesce listener events into a
// single microtask-deferred read so the setState is dispatched outside the emit
// call stack and repeated events between paints collapse into one update.
const handleViewportChange = () => {
updateViewport();
if (scheduled) {
return;
}
scheduled = true;
queueMicrotask(() => {
scheduled = false;
if (cancelled) {
return;
}
readViewport();
});
Comment on lines +359 to +365
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@greptile-apps greptile-apps Bot May 8, 2026

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P2 The scheduled = false reset happens unconditionally before the cancelled guard. If readViewport() throws (e.g., the scrollbox is torn down between the microtask being queued and it running), scheduled is already false and a new event could queue a fresh microtask before the thrown exception propagates. Resetting scheduled only on the non-cancelled branch makes the intent clearer and keeps the flag consistent through the full life of the microtask.

Suggested change
queueMicrotask(() => {
scheduled = false;
if (cancelled) {
return;
}
readViewport();
});
queueMicrotask(() => {
if (cancelled) {
scheduled = false;
return;
}
scheduled = false;
readViewport();
});
Prompt To Fix With AI 
This is a comment left during a code review.
Path: src/ui/components/panes/DiffPane.tsx
Line: 359-365

Comment:
The `scheduled = false` reset happens unconditionally before the `cancelled` guard. If `readViewport()` throws (e.g., the scrollbox is torn down between the microtask being queued and it running), `scheduled` is already `false` and a new event could queue a fresh microtask before the thrown exception propagates. Resetting `scheduled` only on the non-cancelled branch makes the intent clearer and keeps the flag consistent through the full life of the microtask.

```suggestion
      queueMicrotask(() => {
        if (cancelled) {
          scheduled = false;
          return;
        }
        scheduled = false;
        readViewport();
      });
```

How can I resolve this? If you propose a fix, please make it concise.

};

updateViewport();
readViewport();
scrollBox.verticalScrollBar.on("change", handleViewportChange);
scrollBox.viewport.on("layout-changed", handleViewportChange);
scrollBox.viewport.on("resized", handleViewportChange);

return () => {
cancelled = true;
scrollBox.verticalScrollBar.off("change", handleViewportChange);
scrollBox.viewport.off("layout-changed", handleViewportChange);
scrollBox.viewport.off("resized", handleViewportChange);
Expand Down