fix: calculate vidxOffset correctly when clientHeight != offsetHeight (#7268) (#7272)

* refactor: extract _maxVirtualIndexOffset getter (#7227)
* fix: calculate vidxOffset correctly when clientHeight != offsetHeight (#7268)

Author: vursen

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

@@ -82,6 +82,10 @@ export class IronListAdapter {
     return this.lastVisibleIndex + this._vidxOffset;
   }
 
+  get _maxVirtualIndexOffset() {
+    return this.size - this._virtualCount;
+  }
+
   __hasPlaceholders() {
     return this.__getVisibleElements().some((el) => el.__virtualizerPlaceholder);
   }
@@ -106,7 +110,7 @@ export class IronListAdapter {
     let targetVirtualIndex = Math.floor((index / this.size) * this._virtualCount);
     if (this._virtualCount - targetVirtualIndex < visibleElementCount) {
       targetVirtualIndex = this._virtualCount - (this.size - index);
-      this._vidxOffset = this.size - this._virtualCount;
+      this._vidxOffset = this._maxVirtualIndexOffset;
     } else if (targetVirtualIndex < visibleElementCount) {
       if (index < OFFSET_ADJUST_MIN_THRESHOLD) {
         targetVirtualIndex = index;
@@ -719,15 +723,16 @@ export class IronListAdapter {
 
   /** @private */
   _adjustVirtualIndexOffset(delta) {
+    const maxOffset = this._maxVirtualIndexOffset;
+
     if (this._virtualCount >= this.size) {
       this._vidxOffset = 0;
     } else if (this.__skipNextVirtualIndexAdjust) {
       this.__skipNextVirtualIndexAdjust = false;
     } else if (Math.abs(delta) > 10000) {
       // Process a large scroll position change
-      const scale = this._scrollTop / (this.scrollTarget.scrollHeight - this.scrollTarget.offsetHeight);
-      const offset = scale * this.size;
-      this._vidxOffset = Math.round(offset - scale * this._virtualCount);
+      const scale = this._scrollTop / (this.scrollTarget.scrollHeight - this.scrollTarget.clientHeight);
+      this._vidxOffset = Math.round(scale * maxOffset);
     } else {
       // Make sure user can always swipe/wheel scroll to the start and end
       const oldOffset = this._vidxOffset;
@@ -746,7 +751,6 @@ export class IronListAdapter {
       }
 
       // Near end
-      const maxOffset = this.size - this._virtualCount;
       if (this._scrollTop >= this._maxScrollTop && this._maxScrollTop > 0) {
         this._vidxOffset = maxOffset;
         if (oldOffset !== this._vidxOffset) {

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

@@ -102,6 +102,16 @@ describe('unlimited size', () => {
     expect(item.getBoundingClientRect().top).to.equal(scrollTarget.getBoundingClientRect().top);
   });
 
+  it('should manually scroll to end when the scroll target has a border', async () => {
+    scrollTarget.style.borderTop = '20px solid black';
+
+    scrollTarget.scrollTop = scrollTarget.scrollHeight;
+    await oneEvent(scrollTarget, 'scroll');
+
+    const item = elementsContainer.querySelector(`#item-${virtualizer.size - 1}`);
+    expect(item.getBoundingClientRect().bottom).to.be.closeTo(scrollTarget.getBoundingClientRect().bottom, 1);
+  });
+
   it('should manually scroll to start after scroll to index', async () => {
     virtualizer.scrollToIndex(virtualizer.size / 400);