fix(cdk/scrolling): content jumping in appendOnly mode (#25097)

We were creating the `transform` string before adjusting the offset for `appendOnly` mode which was causing the list to jump down before being reset on the next scroll event.

These changes resolve the issue by moving the offset adjustment up to before the `transform`.

Fixes #25077.

src/cdk/scrolling/virtual-scroll-viewport.spec.ts

@@ -1030,6 +1030,7 @@ describe('CdkVirtualScrollViewport', () => {
     let fixture: ComponentFixture<VirtualScrollWithAppendOnly>;
     let testComponent: VirtualScrollWithAppendOnly;
     let viewport: CdkVirtualScrollViewport;
+    let contentWrapperEl: HTMLElement;
 
     beforeEach(waitForAsync(() => {
       TestBed.configureTestingModule({
@@ -1039,6 +1040,9 @@ describe('CdkVirtualScrollViewport', () => {
       fixture = TestBed.createComponent(VirtualScrollWithAppendOnly);
       testComponent = fixture.componentInstance;
       viewport = testComponent.viewport;
+      contentWrapperEl = fixture.nativeElement.querySelector(
+        '.cdk-virtual-scroll-content-wrapper',
+      ) as HTMLElement;
     }));
 
     it('should not remove item that have already been rendered', fakeAsync(() => {
@@ -1085,6 +1089,21 @@ describe('CdkVirtualScrollViewport', () => {
 
       expect(viewport.getOffsetToRenderedContentStart()).toBe(0);
     }));
+
+    it('should not set a transform when scrolling', fakeAsync(() => {
+      finishInit(fixture);
+      triggerScroll(viewport, 0);
+      fixture.detectChanges();
+      flush();
+
+      expect(contentWrapperEl.style.transform).toBe('translateY(0px)');
+
+      triggerScroll(viewport, testComponent.itemSize * 10);
+      fixture.detectChanges();
+      flush();
+
+      expect(contentWrapperEl.style.transform).toBe('translateY(0px)');
+    }));
   });
 
   describe('with custom scrolling element', () => {

src/cdk/scrolling/virtual-scroll-viewport.ts

@@ -333,15 +333,16 @@ export class CdkVirtualScrollViewport extends CdkVirtualScrollable implements On
    * (in pixels).
    */
   setRenderedContentOffset(offset: number, to: 'to-start' | 'to-end' = 'to-start') {
+    // In appendOnly, we always start from the top
+    offset = this.appendOnly && to === 'to-start' ? 0 : offset;
+
     // For a horizontal viewport in a right-to-left language we need to translate along the x-axis
     // in the negative direction.
     const isRtl = this.dir && this.dir.value == 'rtl';
     const isHorizontal = this.orientation == 'horizontal';
     const axis = isHorizontal ? 'X' : 'Y';
     const axisDirection = isHorizontal && isRtl ? -1 : 1;
     let transform = `translate${axis}(${Number(axisDirection * offset)}px)`;
-    // in appendOnly, we always start from the top
-    offset = this.appendOnly && to === 'to-start' ? 0 : offset;
     this._renderedContentOffset = offset;
     if (to === 'to-end') {
       transform += ` translate${axis}(-100%)`;