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

首先复现 issue 中所示的 mat-expansion-panel 演示行为，其中多个面板处于折叠状态，一个面板处于展开状态。跟踪 mat-expansion-panel 的实现及其动画行为；完成标准是折叠面板的内容从 DOM 中移除，同时不破坏过渡效果。

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