bug(cdk-virtual-scroll): Scrolling fast makes the viewport blank until scrolling stops

#### Reproduction

https://stackblitz.com/edit/angular-qxcwbr

Steps to reproduce:
There are two virtual scroll viewports - the first has the problem. Take the scroll bar with the mouse and scroll down fast. The whole viewport will be blank as long as you scroll.
The second viewport uses a bizarre fix and if you try the same it will magically work.

#### Expected Behavior

Both of the viewports should scroll without viewport going blank

#### Actual Behavior

Without the bizarre fix the viewport becomes blank while fast scrolling

#### Environment

- Angular: 10.2.2
- CDK/Material: 10.2.7
- Browser(s): Chrome Version 86.0.4240.111 (Official Build) (64-bit)
- Operating System (e.g. Windows, macOS, Ubuntu): Windows 10

Start with the linked StackBlitz reproduction and confirm the blank viewport during fast scrollbar movement. Then trace the Angular CDK virtual-scroll implementation and compare the two viewports, including the one using the reported workaround. Done means both viewports remain populated while scrolling quickly, with regression coverage for the reproduced behavior.

Written by the indexing model from the issue text.