fix: update scroll container height on virtualizer size change (#12267) (#12268)

This PR cherry-picks changes from the original PR #12267 to branch 25.2.

---

#### Original PR description
> When the virtualizer size changes, the scroll container height needs
to be updated. This used to happen reliably only because the virtualizer
called `scrollToIndex` unconditionally in that situation, and that call
force-updated the height as a side effect. However, that call was made
conditional in #11196, and
no longer happens when no scroll position needs to be restored. Without
it, the amortization logic in `_updateScrollerSize` can skip the height
update if the change is smaller than the viewport, so the scroll
container ends up with a stale height.
> 
> This PR fixes that by making the size setter force the height update
with an explicit `_updateScrollerSize(true)` call. This also covers the
`allRowsVisible` growth scenario from #12146, so the complexity
introduced there is no longer needed and has been removed. The
regression tests from that PR still pass.
> 
> Fixes vaadin/flow-components#9802

Co-authored-by: Sergey Vinogradov <mr.vursen@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: 3 people

packages/component-base/src/virtualizer-iron-list-adapter.js

@@ -22,7 +22,6 @@ export class IronListAdapter {
     reorderElements,
     elementsContainer,
     __disableHeightPlaceholder,
-    __alwaysUpdateScrollerSize,
   }) {
     this.isAttached = true;
     this._vidxOffset = 0;
@@ -38,12 +37,6 @@ export class IronListAdapter {
     // elements with a non-zero height. Not for public use.
     this.__disableHeightPlaceholder = __disableHeightPlaceholder ?? false;
 
-    // Internal option: a predicate that, when it returns true, makes the scroller
-    // height always be applied instead of amortized (see `_updateScrollerSize`).
-    // Used by components whose height tracks the content exactly (e.g. the grid's
-    // `allRowsVisible` mode). Not for public use.
-    this.__alwaysUpdateScrollerSize = __alwaysUpdateScrollerSize;
-
     // Iron-list uses this value to determine how many pages of elements to render
     this._maxPages = 1.3;
 
@@ -223,11 +216,6 @@ export class IronListAdapter {
     this.__afterElementsUpdated(updatedElements);
   }
 
-  /** @override */
-  _updateScrollerSize(forceUpdate) {
-    super._updateScrollerSize(forceUpdate || !!this.__alwaysUpdateScrollerSize?.());
-  }
-
   /**
    * Updates the height for a given set of items.
    *
@@ -426,6 +414,8 @@ export class IronListAdapter {
       requestAnimationFrame(() => this._resizeHandler());
     }
 
+    this._updateScrollerSize(true);
+
     // Re-render items once the scroll position has been restored.
     // This call also updates the cached scrollTarget height and
     // rechecks whether more virtual elements are needed, since the

packages/component-base/test/virtualizer.test.js

@@ -234,6 +234,14 @@ describe('virtualizer', () => {
     expect(item.getBoundingClientRect().top).to.be.closeTo(scrollTarget.getBoundingClientRect().top - 10, 1);
   });
 
+  it('should update scroll container height on size change', () => {
+    virtualizer.size += 5;
+    expect(elementsContainer.offsetHeight).to.equal(virtualizer.size * 30);
+
+    virtualizer.size -= 5;
+    expect(elementsContainer.offsetHeight).to.equal(virtualizer.size * 30);
+  });
+
   it('should not call updateElement when size increase does not affect visible indexes', () => {
     const updateElement = sinon.spy((el, index) => {
       el.textContent = `item-${index}`;

packages/grid/src/vaadin-grid-mixin.js

@@ -247,13 +247,6 @@ export const GridMixin = (superClass) =>
         // otherwise be triggered by this logic because it reads the row height
         // right after updating the rows' content.
         __disableHeightPlaceholder: true,
-        // The virtualizer amortizes scroller height updates to avoid reflows while
-        // scrolling. In `allRowsVisible` mode the grid has no scrolling and its
-        // height must track the content exactly, so tell the virtualizer to always
-        // apply the scroller height. Otherwise the items container can be left at a
-        // stale, too-small height and clip rows when the grid grows (e.g. when
-        // expanding a tree grid from a small size).
-        __alwaysUpdateScrollerSize: () => this.allRowsVisible,
       });
 
       this._tooltipController = new TooltipController(this);