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

Inizia riproducendo il comportamento della demo di mat-expansion-panel mostrata nell’issue, con diversi pannelli compressi e uno espanso. Traccia l’implementazione di mat-expansion-panel e il comportamento della relativa animazione; il lavoro è completato quando i contenuti dei pannelli compressi vengono rimossi dal DOM senza interrompere le transizioni.

Scritto dal modello di indicizzazione a partire dal testo della issue.