perf(virtual-scroll): improve scrolling performance (#12490)

Adds contain and translateZ to viewport and wrapper CSS and switches spacer to use transform instead of width/height

Author: mmalerba

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

@@ -9,7 +9,4 @@
   Spacer used to force the scrolling container to the correct size for the *total* number of items
   so that the scrollbar captures the size of the entire data set.
 -->
-<div class="cdk-virtual-scroll-spacer"
-     [style.height.px]="orientation === 'horizontal' ? 1 : _totalContentSize"
-     [style.width.px]="orientation === 'horizontal' ? _totalContentSize : 1">
-</div>
+<div class="cdk-virtual-scroll-spacer" [style.transform]="_totalContentSizeTransform"></div>

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

@@ -32,6 +32,9 @@ cdk-virtual-scroll-viewport {
   display: block;
   position: relative;
   overflow: auto;
+  contain: strict;
+  transform: translateZ(0);
+  will-change: scroll-position;
 }
 
 // Wrapper element for the rendered content. This element will be transformed to push the rendered
@@ -40,7 +43,8 @@ cdk-virtual-scroll-viewport {
   position: absolute;
   top: 0;
   left: 0;
-  will-change: contents, transform;
+  contain: content;
+  will-change: transform;
 }
 
 .cdk-virtual-scroll-orientation-horizontal .cdk-virtual-scroll-content-wrapper {
@@ -57,5 +61,17 @@ cdk-virtual-scroll-viewport {
 // set if it were rendered all at once. This ensures that the scrollable content region is the
 // correct size.
 .cdk-virtual-scroll-spacer {
-  will-change: height, width;
+  position: absolute;
+  top: 0;
+  left: 0;
+  height: 1px;
+  width: 1px;
+  transform-origin: 0 0;
+  will-change: transform;
+
+  [dir='rtl'] & {
+    right: 0;
+    left: auto;
+    transform-origin: 100% 0;
+  }
 }

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

@@ -77,10 +77,16 @@ export class CdkVirtualScrollViewport implements OnInit, OnDestroy {
   /** A stream that emits whenever the rendered range changes. */
   renderedRangeStream: Observable<ListRange> = this._renderedRangeSubject.asObservable();
 
+  /**
+   * The transform used to scale the spacer to the same size as all content, including content that
+   * is not currently rendered.
+   */
+  _totalContentSizeTransform = '';
+
   /**
    * The total size of all content (in pixels), including content that is not currently rendered.
    */
-  _totalContentSize = 0;
+  private _totalContentSize = 0;
 
   /**
    * The CSS transform applied to the rendered subset of items so that they appear within the bounds
@@ -211,6 +217,8 @@ export class CdkVirtualScrollViewport implements OnInit, OnDestroy {
   setTotalContentSize(size: number) {
     if (this._totalContentSize !== size) {
       this._totalContentSize = size;
+      const axis = this.orientation == 'horizontal' ? 'X' : 'Y';
+      this._totalContentSizeTransform = `scale${axis}(${this._totalContentSize})`;
       this._markChangeDetectionNeeded();
     }
   }