fix(virtual-list): keep the focus snap when a scroll request echoes it

.changeset/virtuallist-focus-echo-resnap.md

@@ -0,0 +1,5 @@
+---
+'@plextv/react-lightning-components': patch
+---
+
+virtual list keeps the focused row snapped when a scrollToIndex or scrollToOffset asks for the spot focus already scrolled to, so rows above it resizing afterwards no longer leave it off its rest position.

packages/react-lightning-components/src/components/VirtualList/useScrollHandler.spec.tsx

@@ -0,0 +1,111 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+import type { LightningElement } from '@plextv/react-lightning';
+
+import { createTestRenderer } from './createTestRenderer';
+import type { LayoutManager } from './LayoutManager';
+import { type UseScrollHandlerResult, useScrollHandler } from './useScrollHandler';
+
+const ROW = 100;
+const COUNT = 20;
+const VIEWPORT = 300;
+
+function setup() {
+  const { render } = createTestRenderer();
+  const sizes = Array.from({ length: COUNT }, () => ROW);
+  const offsetOf = (index: number) => sizes.slice(0, index).reduce((sum, size) => sum + size, 0);
+  const totalSize = () => offsetOf(COUNT);
+  const layoutManager = {
+    getLayout: (index: number) => ({
+      offset: offsetOf(index),
+      size: sizes[index],
+      column: 0,
+      crossOffset: 0,
+      crossSize: 500,
+    }),
+    get totalSize() {
+      return totalSize();
+    },
+  } as unknown as LayoutManager<unknown>;
+  let current: UseScrollHandlerResult | undefined;
+
+  function Probe({ total }: { total: number }) {
+    current = useScrollHandler({
+      layoutManager,
+      horizontal: false,
+      viewportSize: VIEWPORT,
+      itemAreaOffset: 0,
+      totalContentSize: total,
+      viewportCrossSize: 500,
+      totalCrossSize: 500,
+      animationDuration: 300,
+      snapToAlignment: 'start',
+      onEndReachedThreshold: null,
+      paddingStart: 0,
+      paddingEnd: 0,
+      itemCount: COUNT,
+      keepFocusedItemSnapped: true,
+    });
+
+    return null;
+  }
+
+  const rerender = () => render(<Probe total={totalSize()} />);
+
+  rerender();
+
+  // oxlint-disable-next-line typescript/no-non-null-assertion -- set by the first render
+  const handler = () => current!;
+
+  handler().contentRef.current = { node: { x: 0, y: 0 } } as unknown as LightningElement;
+
+  const focus = (index: number) => {
+    const child = {
+      getRelativePosition: () => ({ x: 0, y: offsetOf(index) }),
+      node: { w: 500, h: sizes[index] },
+      props: {},
+      children: [],
+    } as unknown as LightningElement;
+
+    handler().handleChildFocused(child, index);
+    rerender();
+  };
+
+  const resize = (index: number, size: number) => {
+    sizes[index] = size;
+    rerender();
+  };
+
+  return { handler, focus, resize, rerender };
+}
+
+describe('useScrollHandler', () => {
+  beforeEach(() => {
+    vi.stubGlobal('requestAnimationFrame', () => 1);
+    vi.stubGlobal('cancelAnimationFrame', () => {});
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it('keeps the focused row snapped when a scrollToIndex echoes the focus scroll', () => {
+    const { handler, focus, resize } = setup();
+
+    focus(10);
+    handler().scrollToIndex(10, true);
+    resize(5, 76);
+
+    expect(handler().scrollOffsetRef.current).toBe(976);
+  });
+
+  it('lets a scroll to another row release the focused row', () => {
+    const { handler, focus, resize } = setup();
+
+    focus(10);
+    handler().scrollToIndex(12, true);
+    resize(5, 76);
+
+    expect(handler().scrollOffsetRef.current).toBe(1200);
+  });
+});

packages/react-lightning-components/src/components/VirtualList/useScrollHandler.ts

@@ -361,7 +361,14 @@ export function useScrollHandler(options: UseScrollHandlerOptions): UseScrollHan
   }
 
   function scrollToOffset(offset: number, animated = true): void {
-    focusAnchorRef.current = null;
+    const anchor = focusAnchorRef.current;
+
+    // A request for the spot the focused item already snaps to (a focus handler
+    // echoing the list's own scroll) keeps the item snapped as rows above resize.
+    if (!anchor || Math.abs(clamp(offset) - clamp(resolveFocusTarget(anchor))) >= 1) {
+      focusAnchorRef.current = null;
+    }
+
     indexIntentRef.current = null;
     scrollTo(offset, animated, true);
   }
@@ -388,7 +395,11 @@ export function useScrollHandler(options: UseScrollHandlerOptions): UseScrollHan
     }
 
     scrollToOffset(target, animated);
-    indexIntentRef.current = target;
+
+    // A kept focus anchor already follows the item; a fixed intent would pull it back.
+    if (!focusAnchorRef.current) {
+      indexIntentRef.current = target;
+    }
   }
 
   function scrollToEnd(animated = true): void {