Overlays behave erratically when using Virtual Scrolling

#### What is the expected behavior?
Users should be able to utilize Virtual Scrolling with the Tree without worrying about other functionality breaking.

#### What is the current behavior?
When using Tooltips on Tree Nodes in combination with Virtual Scrolling, the Tooltips appear improperly without context.

#### What are the steps to reproduce?
Apply Virtual Scrolling to a Tree and then expose Tooltips on the Tree Node Definitions.

[Demo](https://stackblitz.com/edit/simplified-material-tree-with-virtual-scroll-xkvghx?file=app%2Ftree-flat-overview-example.html) - Mousing over any of the items in the Flat Tree should display a Tool Tip containing the Node's name, seemingly randomly it does accurately display the name in the proper place, most of the time you're left with a small gray box.

Edit: I have updated the demo to include a behavior where clicking on the text of a node will display a Snack Bar containing the node's name. This is also appearing as a gray bar at the top of the screen in many situations. This seems to be related to overlays as a whole in this context.

Beginne damit, die verlinkte StackBlitz-Demo auszuführen und die Tooltip- und Snack-bar-Fehler mit einem Angular Material Tree unter Verwendung von Virtual Scrolling zu reproduzieren. Verfolge, wie sich die Overlays der Tree-Knoten verhalten, während Elemente recycelt werden; abgeschlossen ist die Aufgabe, wenn Tooltips den richtigen Knotennamen an der richtigen Stelle anzeigen und die Snack Bar normal erscheint.

Vom Indexierungsmodell aus dem Issue-Text verfasst.