feat(Expansion Panel): "Virtual" expansion panel

[Goldman90]

Feature Description

for certain use cases, an option to make expansion panel not render the items inside 'em when closed(like a virtual scroll) would be really useful

Use Case

for example, in the image i've got two expansion panels, each with 50 components inside, if i closed one, those 50 components would still be loaded in the page, unnecesarily

[crisbeto]

This sounds like the "Lazy rendering" use case from the docs: https://material.angular.io/components/expansion/overview#lazy-rendering

[Goldman90]

my bad, though it was as you said, but lazy renedring will keep the rendered elements after they were rendered once, instead of removing 'em if it's closed again, as a virtual scroll would

[crisbeto]

I believe the thinking behind keeping the elements in the DOM after the first render is that you've already paid the cost for rendering the elements so keeping them for the next time it's opened is cheaper than removing and re-adding them. That sort of breaks down for virtual scrolling when you have tons of elements.

[Goldman90]

the problem is that, in cases like the one described in my first post, when all elements have been rendered, #8287 happens

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