MatExpansionPanel open animation

[e-thn]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

When the panel is opened, the content should be revealed gradually.

What is the current behavior?

Currently, once the opening sequence is initiated, the content is shown immediately, then the animation follows.

What are the steps to reproduce?

https://stackblitz.com/angular/pdgrgaapeoo
Just open the expansion panel! If you watch closely you can see it, but here is a screenshot of what is going on right when you click the button:

What is the use-case or motivation for changing an existing behavior?

Seems to me like this isn't intended behaviour for the animation, if it is, my bad.

[jstuth]

I got the same issue!
But the closing animation is properly clipping the content of the MatExpensionPanel.

[e-thn]

Yes, same here. It's something to do with the overflow property of the content.

[josephperrott]

This will be fixed by #8820 as it has the same underlying cause as #8754

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