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

从链接的 StackBlitz 复现开始，确认快速移动滚动条时视口为空。然后跟踪 Angular CDK virtual-scroll 的实现，并比较两个视口，包括使用已报告 workaround 的视口。完成的标准是快速滚动时两个视口都保持有内容，并且针对复现行为有回归测试覆盖。

由索引模型根据 Issue 内容生成。