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

Start by reproducing the behavior in the mat-expansion-panel demo shown in the issue, with several panels collapsed and one expanded. Trace the mat-expansion-panel implementation and its animation behavior; done means collapsed panel contents are removed from the DOM without breaking transitions.

Written by the indexing model from the issue text.