mat-expansion-panel keeps contents of collapsed panels in DOM

I found this while debugging rendering performance in my app. I have a large section that initially renders as a big mat-expansion-panel with all panels collapsed, except one. Here is a screenshot of mat-expansion-panel demo that sxhibits the same behavior:


I believe that the contents of the collapsed sections should be removed from the DOM to not slow down the application by rendering content that isn't currently visible.

If the reason for doing that was animations, then the animations probably need fixing - Angular supports keeping things in DOM while they are animating. Or maybe this was used, but the animation end never gets registered? I'm gussing :) In any case I believe it's a bug.

CC @jelbourn

Possilbly related (?): https://github.com/angular/components/issues/8287

Beginne damit, das Verhalten in der im Issue gezeigten mat-expansion-panel-Demo nachzustellen, wobei mehrere Panels eingeklappt und eines ausgeklappt ist. Verfolge die Implementierung von mat-expansion-panel und das Verhalten seiner Animation; abgeschlossen ist die Aufgabe, wenn die Inhalte eingeklappter Panels aus dem DOM entfernt werden, ohne die Übergänge zu beeinträchtigen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.