fix: preserve large items when new items are added

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

@@ -727,6 +727,53 @@ export class IronListAdapter {
     }
   }
 
+  /**
+   * An optimal physical size such that we will have enough physical items
+   * to fill up the viewport and recycle when the user scrolls.
+   *
+   * This default value assumes that we will at least have the equivalent
+   * to a viewport of physical items above and below the user's viewport.
+   * @override
+   */
+  get _optPhysicalSize() {
+    const optPhysicalSize = super._optPhysicalSize;
+    // No need to adjust
+    if (optPhysicalSize <= 0 || this.__hasPlaceholders()) {
+      return optPhysicalSize;
+    }
+    // Item height buffer accounts for the cases where some items are much larger than the average.
+    // This can lead to some items not being rendered and leaving empty space in the viewport.
+    // https://github.com/vaadin/flow-components/issues/6651
+    return optPhysicalSize + this.__getItemHeightBuffer();
+  }
+
+  /**
+   * Extra item height buffer used when calculating optimal physical size.
+   *
+   * The iron list core uses the optimal physical size when determining whether to increase the item pool.
+   * For the cases where some items are much larger than the average, the iron list core might not increase item pool.
+   * This can lead to the large item not being rendered.
+   *
+   * @returns {Number} - Extra item height buffer
+   * @private
+   */
+  __getItemHeightBuffer() {
+    // No need for a buffer with no items
+    if (this._physicalCount === 0) {
+      return 0;
+    }
+    // The regular buffer zone height for either top or bottom
+    const bufferZoneHeight = Math.ceil((this._viewportHeight * (this._maxPages - 1)) / 2);
+    // The maximum height of the currently rendered items
+    const maxItemHeight = Math.max(...this._physicalSizes);
+    // Only add buffer if the item is larger that the other items
+    if (maxItemHeight > Math.min(...this._physicalSizes)) {
+      // Add a buffer height since the large item can still be in the viewport and out of the original buffer
+      return Math.max(0, maxItemHeight - bufferZoneHeight);
+    }
+    return 0;
+  }
+
   /**
    * @returns {Number|undefined} - The browser's default font-size in pixels
    * @private

packages/component-base/test/virtualizer-variable-row-height.test.js

@@ -95,6 +95,19 @@ describe('virtualizer - variable row height - large variance', () => {
     }
   }
 
+  /**
+   * Sets a large size, and expands the first and the last items.
+   */
+  function initWithLargeSize() {
+    // This is kept separate because some tests need physical items
+    // to be initialized during the test.
+    virtualizer.size = 200000;
+    // Expand the first and the last item
+    expandedItems = [0, virtualizer.size - 1];
+    virtualizer.update();
+    virtualizer.scrollToIndex(0);
+  }
+
   beforeEach(() => {
     scrollTarget = fixtureSync(`
       <div style="height: 300px; width: 200px;">
@@ -128,16 +141,10 @@ describe('virtualizer - variable row height - large variance', () => {
     });
 
     sinon.spy(virtualizer.__adapter, '__fixInvalidItemPositioning');
-
-    virtualizer.size = 200000;
-
-    // Expand the first and the last item
-    expandedItems = [0, virtualizer.size - 1];
-    virtualizer.update();
-    virtualizer.scrollToIndex(0);
   });
 
   it('should reveal new items when scrolling downwards', async () => {
+    initWithLargeSize();
     const rect = scrollTarget.getBoundingClientRect();
 
     await scrollDownwardsFromStart();
@@ -155,6 +162,7 @@ describe('virtualizer - variable row height - large variance', () => {
   });
 
   it('should reveal new items when scrolling upwards', async () => {
+    initWithLargeSize();
     const rect = scrollTarget.getBoundingClientRect();
 
     await scrollToEnd();
@@ -173,6 +181,7 @@ describe('virtualizer - variable row height - large variance', () => {
   });
 
   it('should not update the item at last index', async () => {
+    initWithLargeSize();
     updateElement.resetHistory();
     await scrollDownwardsFromStart();
     await fixItemPositioningTimeout();
@@ -185,6 +194,7 @@ describe('virtualizer - variable row height - large variance', () => {
   });
 
   it('should not update the item at first index', async () => {
+    initWithLargeSize();
     await scrollToEnd();
     updateElement.resetHistory();
     await scrollUpwardsFromEnd();
@@ -197,6 +207,7 @@ describe('virtualizer - variable row height - large variance', () => {
   });
 
   it('should allow scrolling to end of a padded scroll target', async () => {
+    initWithLargeSize();
     scrollTarget.style.padding = '60px 0 ';
     scrollTarget.style.boxSizing = 'border-box';
 
@@ -208,6 +219,7 @@ describe('virtualizer - variable row height - large variance', () => {
   });
 
   it('should allow scrolling to start', async () => {
+    initWithLargeSize();
     // Expand every 10th item
     expandedItems = Array.from(Array(virtualizer.size).keys()).filter((index) => index % 10 === 0);
 
@@ -221,17 +233,39 @@ describe('virtualizer - variable row height - large variance', () => {
   });
 
   it('should not jam when when size is changed after fix', async () => {
+    initWithLargeSize();
     await scrollDownwardsFromStart();
     await fixItemPositioningTimeout();
     virtualizer.size = 1;
   });
 
   it('should not invoke when size is changed after scrolling', async () => {
+    initWithLargeSize();
     await scrollDownwardsFromStart();
     virtualizer.size = 0;
     await fixItemPositioningTimeout();
     expect(virtualizer.__adapter.__fixInvalidItemPositioning.callCount).to.equal(0);
   });
+
+  it('should preserve large item in viewport when new item is added', async () => {
+    // Initially only have a large item
+    virtualizer.size = 1;
+    expandedItems = [0];
+    virtualizer.update();
+    await nextFrame();
+
+    // Calculated using the virtualizer height of 300px and regular item height of 30px
+    const requiredRegularItemCountToFillViewport = 10;
+
+    // Add items until the large item disappears
+    for (let i = 0; i < requiredRegularItemCountToFillViewport - 1; i++) {
+      // Add a new item and scroll to the end
+      virtualizer.size += 1;
+      virtualizer.scrollToIndex(Infinity);
+      await nextFrame();
+      expect(virtualizer.firstVisibleIndex).to.equal(0);
+    }
+  });
 });
 
 describe('virtualizer - variable row height - size changes', () => {