bug(DragDrop): Drag&Drop not working inside MatDialog and when the page's height is too big

#### Reproduction

Here is a reproduction. There is a button down there which opens a dialog with draggable items.

https://stackblitz.com/edit/angular-9-material-starter-lktwr2?file=src/app/app.component.css

#### Expected Behavior

Drag&Drop should work.

#### Actual Behavior

I have a project where I have a table with a lot of items, and when I scroll down (for example to the middle of the table, page height ~is 4000px), and by clicking on a row I open a dialog where I want to use drag&drop and where it doesn't work (as in the reproduction). But if I click on a row which is at the beginning of the table, drag&drop is working fine.

#### Environment

In the reproduction I used Angular 9, but I see this in Angular 11 as well. Not sure if this fixed in 12th version.

从链接的 StackBlitz 复现开始，重点查看 src/app/app.component.css，并在页面滚动后复现对话框内的拖放失败问题。比较表格顶部和中部的行为；当可拖动项目无论页面高度如何都能在 MatDialog 中正常工作，并且复现内容在 Angular 9 和 11 中仍能正确运行时，即表示完成。

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