v12 Virtualscroll header flicks

[SarmadAljazrawi]

Hi,
Having issues with Virtualscroll where header flicks/blinks when scrolling. Its because cdk-virtual-scroll-viewport is wrapping the table. This issue was not in v11 as cdk-virtuals-scroll-viewport was under the table.

A quick solution to solve the problem is to move the "thead" tag outside the cdk-virtual-scroll-viewport wrapper. Here is a link to show https://ibb.co/VLqw2x0