fix(cdk/virtual-scroll): fix subpixel rounding errors on hdpi screens

Using `transform` appears to cause rounding errors that can accumulate and
become significant after scrolling through a large number of items.
Switching to `width` / `height` corrects this.

Author: mmalerba

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

@@ -9,4 +9,6 @@
   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.transform]="_totalContentSizeTransform"></div>
+<div class="cdk-virtual-scroll-spacer"
+     [style.width]="orientation == 'horizontal' ? _totalContentSize + 'px' : ''"
+     [style.height]="orientation == 'vertical' ? _totalContentSize + 'px' : ''"></div>

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

@@ -89,16 +89,10 @@ export class CdkVirtualScrollViewport extends CdkScrollable implements OnInit, O
   /** 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.
    */
-  private _totalContentSize = 0;
+  _totalContentSize = 0;
 
   /**
    * The CSS transform applied to the rendered subset of items so that they appear within the bounds
@@ -238,8 +232,6 @@ export class CdkVirtualScrollViewport extends CdkScrollable implements OnInit, O
   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();
     }
   }

tools/public_api_guard/cdk/scrolling.d.ts

@@ -90,7 +90,7 @@ export declare type CdkVirtualForOfContext<T> = {
 
 export declare class CdkVirtualScrollViewport extends CdkScrollable implements OnInit, OnDestroy {
     _contentWrapper: ElementRef<HTMLElement>;
-    _totalContentSizeTransform: string;
+    _totalContentSize: number;
     elementRef: ElementRef<HTMLElement>;
     orientation: 'horizontal' | 'vertical';
     renderedRangeStream: Observable<ListRange>;