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fix(ScrollArea): reset scroll only on direct viewport child swaps (#1349) #2045

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fix(ScrollArea): reset scroll only on direct viewport child swaps (#1349) #2045

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47451a7
fix(ScrollArea): reset scroll only on direct viewport child swaps (#1…
kotAPI Jun 24, 2026
a68492b
fix(ScrollArea): re-sync ResizeObserver targets after viewport child …
kotAPI Jun 24, 2026
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34 changes: 30 additions & 4 deletions src/components/ui/ScrollArea/fragments/ScrollAreaRoot.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -53,16 +53,42 @@ const ScrollAreaRoot = forwardRef<ScrollAreaRootElement, ScrollAreaRootProps>(({
};

const resizeObserver = new ResizeObserver(() => handleResize());
resizeObserver.observe(viewport);
Array.from(viewport.children).forEach(child => resizeObserver.observe(child));
const syncResizeObservers = () => {
resizeObserver.disconnect();
resizeObserver.observe(viewport);
Array.from(viewport.children).forEach(child => {
if (child instanceof Element) {
resizeObserver.observe(child);
}
});
};

syncResizeObservers();

const mutationObserver = new MutationObserver((mutations) => {
const directChildSwap = mutations.some(
(mutation) =>
mutation.type === 'childList'
&& mutation.target === viewport
&& (mutation.addedNodes.length > 0 || mutation.removedNodes.length > 0)
);
Comment on lines +69 to +74

@coderabbitai coderabbitai Bot Jun 24, 2026

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🎯 Functional Correctness | 🟠 Major | ⚡ Quick win

Narrow the reset predicate to actual child replacements.

directChildSwap currently fires for any direct child add/remove, so appending or removing an item directly under the viewport will reset the user to the top. Track added and removed nodes across the batch and reset only when both occur for the viewport.

🐛 Proposed predicate tightening 
-            const directChildSwap = mutations.some(
-                (mutation) =>
-                    mutation.type === 'childList'
-                    && mutation.target === viewport
-                    && (mutation.addedNodes.length > 0 || mutation.removedNodes.length > 0)
-            );
+            const viewportChildMutations = mutations.filter(
+                mutation => mutation.type === 'childList' && mutation.target === viewport
+            );
+            const directChildSwap =
+                viewportChildMutations.some(mutation => mutation.addedNodes.length > 0)
+                && viewportChildMutations.some(mutation => mutation.removedNodes.length > 0);
📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
const directChildSwap = mutations.some(
(mutation) =>
mutation.type === 'childList'
&& mutation.target === viewport
&& (mutation.addedNodes.length > 0 || mutation.removedNodes.length > 0)
);
const viewportChildMutations = mutations.filter(
mutation => mutation.type === 'childList' && mutation.target === viewport
);
const directChildSwap =
viewportChildMutations.some(mutation => mutation.addedNodes.length > 0)
&& viewportChildMutations.some(mutation => mutation.removedNodes.length > 0);
🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@src/components/ui/ScrollArea/fragments/ScrollAreaRoot.tsx` around lines 69 -
74, The direct reset predicate in ScrollAreaRoot is too broad because it treats
any childList change on the viewport as a swap. Update the mutation handling
around directChildSwap to inspect the full mutations batch and only trigger a
reset when the viewport has both addedNodes and removedNodes, indicating an
actual child replacement. Keep the logic localized to the viewport mutation
check in ScrollAreaRoot.


if (directChildSwap) {
viewport.scrollTop = 0;
viewport.scrollLeft = 0;
syncResizeObservers();
}

const mutationObserver = new MutationObserver(() => {
handleResize();

if (directChildSwap) {
handleScroll();
}
});

mutationObserver.observe(viewport, {
childList: true,
subtree: true
subtree: false
});

window.addEventListener('resize', handleResize);
Expand Down
40 changes: 39 additions & 1 deletion src/components/ui/ScrollArea/tests/ScrollArea.test.tsx
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
import React from 'react';
import { act, fireEvent, render, screen } from '@testing-library/react';
import { act, fireEvent, render, screen, waitFor } from '@testing-library/react';

import ScrollArea from '../ScrollArea';

Expand Down Expand Up @@ -176,4 +176,42 @@ describe('ScrollArea', () => {

expect(screen.getByTestId('scrollbar')).toHaveAttribute('data-state', 'visible');
});

test('resets scroll when direct viewport child is swapped, not on nested mutations', async() => {
const { rerender } = render(
<ScrollArea.Root style={{ height: 100 }}>
<ScrollArea.Viewport data-testid="viewport" style={{ height: 100, overflow: 'auto' }}>
<div key="panel-a" data-testid="panel-a" style={{ height: 400 }}>Panel A</div>
</ScrollArea.Viewport>
</ScrollArea.Root>
);

const viewport = screen.getByTestId('viewport') as HTMLDivElement;
viewport.scrollTop = 120;

act(() => {
const nested = document.createElement('span');
nested.textContent = 'nested';
screen.getByTestId('panel-a').appendChild(nested);
});

expect(viewport.scrollTop).toBe(120);
Comment on lines +192 to +198

@coderabbitai coderabbitai Bot Jun 24, 2026

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🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Flush the nested mutation before the negative assertion.

MutationObserver callbacks are async, so Line 198 can pass before a broken nested-mutation reset runs. Await a microtask after appending the nested node, then assert the scroll position.

💚 Proposed test hardening 
         act(() => {
             const nested = document.createElement('span');
             nested.textContent = 'nested';
             screen.getByTestId('panel-a').appendChild(nested);
         });
 
+        await act(async() => {
+            await Promise.resolve();
+        });
+
         expect(viewport.scrollTop).toBe(120);
📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
act(() => {
const nested = document.createElement('span');
nested.textContent = 'nested';
screen.getByTestId('panel-a').appendChild(nested);
});
expect(viewport.scrollTop).toBe(120);
act(() => {
const nested = document.createElement('span');
nested.textContent = 'nested';
screen.getByTestId('panel-a').appendChild(nested);
});
await act(async() => {
await Promise.resolve();
});
expect(viewport.scrollTop).toBe(120);


rerender(
<ScrollArea.Root style={{ height: 100 }}>
<ScrollArea.Viewport data-testid="viewport" style={{ height: 100, overflow: 'auto' }}>
<div key="panel-b" data-testid="panel-b" style={{ height: 400 }}>Panel B</div>
</ScrollArea.Viewport>
</ScrollArea.Root>
);

await act(async() => {
await Promise.resolve();
});

const nextViewport = screen.getByTestId('viewport') as HTMLDivElement;
await waitFor(() => {
expect(nextViewport.scrollTop).toBe(0);
});
});
});
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