bug(drag-drop): Unreliable drop-zone detection & markup structure.

[matthewerwin]

Reproduction

Use StackBlitz to reproduce your issue:

https://stackblitz.com/edit/angular-rf6gg6

Steps to reproduce:

1. Look at the stack blitz.
2. Markup has an effect on ability to even start a drag without any console or compile indicator that cdkDrag and cdkDropList cannot be on the same drag element (they could be in v8 --v9 allows them on the same container for the "drop element")
3. Exit/Enter aren't reliably detected based on the client-rect

Expected Behavior

What behavior were you expecting to see?

• Drop zone container should always be detected as soon as the mouse goes over the element.
• If use of cdkDrag/cdkDropList combo only works in certain situations (e.g. codebase looks for this._container on a dragref in v9...which appears to only get set if cdkDropList is on a parent element). That should either be documented and also the compiler should write an error to the console...or the code should be adjusted to set this._container to the element itself to handle the directive combo on the same element.

Actual Behavior

What behavior did you actually see?

• No errors to console or at compile time with directives appended in any fashion.
• v9 does not detect a drop-zone accurately at all for entry or exit. Shaking the mouse a bunch over drop zones or redragging repeatedly will randomly then detect it correctly...though continuing to drag around shows the same inability to detect the drop-zones.

Environment

• Angular: v9.1.2 (really any v9.x for the drop-zone detection)
• CDK/Material: v9.2.1
• Browser(s): 80.0.3987.149
• Operating System (e.g. Windows, macOS, Ubuntu): MacOS 10.14.6

[crisbeto]

Combining with #16671 since it's what I've been using as a catch-all for nested dragging issues. I'll copy over your comment there since it includes a lot of good information.

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