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fix: debounce Group ResizeObserver to prevent iPadOS transient resize corruption #733

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fix: debounce Group ResizeObserver to prevent iPadOS transient resize corruption #733

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124 changes: 73 additions & 51 deletions lib/global/mountGroup.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -40,64 +40,81 @@ export function mountGroup(group: RegisteredGroup) {
const panelIds = new Set<string>();
const separatorIds = new Set<string>();

// Debounce timer to skip transient resize bursts (e.g. iPadOS Safari
// backgrounding fires a rapid sequence of fake resize events, see #731).
let resizeTimer: ReturnType<typeof setTimeout> | null = null;

function handleGroupResize() {
if (!isMounted) {
return;
}

const groupSize = calculateAvailableGroupSize({ group });
if (groupSize === 0) {
// Can't calculate anything meaningful if the group has a width/height of 0
// (This could indicate that it's within a hidden subtree)
return;
}

const groupState = getMountedGroupState(group.id);
if (!groupState) {
// Not mounted yet
return;
}

// Update non-percentage based constraints
const nextDerivedPanelConstraints = calculatePanelConstraints(group);

// Revalidate layout in case constraints have changed or group size changed
const prevLayout = groupState.defaultLayoutDeferred
? calculateDefaultLayout(nextDerivedPanelConstraints)
: groupState.layout;
const unsafeLayout = preserveFixedPanelSizes({
group,
nextGroupSize: groupSize,
prevGroupSize: groupState.groupSize,
prevLayout
});
const nextLayout = validatePanelGroupLayout({
layout: unsafeLayout,
panelConstraints: nextDerivedPanelConstraints
});

if (
!groupState.defaultLayoutDeferred &&
layoutsEqual(groupState.layout, nextLayout) &&
objectsEqual(
groupState.derivedPanelConstraints,
nextDerivedPanelConstraints
) &&
groupState.groupSize === groupSize
) {
return;
}

updateMountedGroup(group, {
defaultLayoutDeferred: false,
derivedPanelConstraints: nextDerivedPanelConstraints,
groupSize,
layout: nextLayout,
separatorToPanels: groupState.separatorToPanels
});
}

// Add Panels with onResize callbacks to ResizeObserver
// Add Group to ResizeObserver also in order to sync % based constraints
const resizeObserver = new ResizeObserver((entries) => {
for (const entry of entries) {
const { borderBoxSize, target } = entry;
if (target === group.element) {
if (isMounted) {
const groupSize = calculateAvailableGroupSize({ group });
if (groupSize === 0) {
// Can't calculate anything meaningful if the group has a width/height of 0
// (This could indicate that it's within a hidden subtree)
return;
}

const groupState = getMountedGroupState(group.id);
if (!groupState) {
// Not mounted yet
return;
}

// Update non-percentage based constraints
const nextDerivedPanelConstraints = calculatePanelConstraints(group);

// Revalidate layout in case constraints have changed or group size changed
const prevLayout = groupState.defaultLayoutDeferred
? calculateDefaultLayout(nextDerivedPanelConstraints)
: groupState.layout;
const unsafeLayout = preserveFixedPanelSizes({
group,
nextGroupSize: groupSize,
prevGroupSize: groupState.groupSize,
prevLayout
});
const nextLayout = validatePanelGroupLayout({
layout: unsafeLayout,
panelConstraints: nextDerivedPanelConstraints
});

if (
!groupState.defaultLayoutDeferred &&
layoutsEqual(groupState.layout, nextLayout) &&
objectsEqual(
groupState.derivedPanelConstraints,
nextDerivedPanelConstraints
) &&
groupState.groupSize === groupSize
) {
return;
}

updateMountedGroup(group, {
defaultLayoutDeferred: false,
derivedPanelConstraints: nextDerivedPanelConstraints,
groupSize,
layout: nextLayout,
separatorToPanels: groupState.separatorToPanels
});
// Debounce to skip transient resize bursts (issue #731).
// On iPadOS Safari, backgrounding the app can fire a rapid sequence
// of resize events with transiently wrong container widths. A short
// debounce ensures only the final settled size is applied.
if (resizeTimer !== null) {
clearTimeout(resizeTimer);
}
resizeTimer = setTimeout(handleGroupResize, 50);

@bvaughn bvaughn Aug 15, 2026

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How did you determine 50ms to be a good debounce interval?

} else {
notifyPanelOnResize(group, target as HTMLElement, borderBoxSize);
}
Expand Down Expand Up @@ -226,6 +243,11 @@ export function mountGroup(group: RegisteredGroup) {
ownerDocument.removeEventListener("pointerup", onDocumentPointerUp, true);
}

if (resizeTimer !== null) {
clearTimeout(resizeTimer);
resizeTimer = null;
}

resizeObserver.disconnect();
};
}