mat-expansion-panel's content shows immediately, outrunning the panel's animation

[kodwi]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

The content inside mat-expansion-panel has to show synchronously with the panel's animation, without outrunning.

What is the current behavior?

The content shows immediately and completly, at the moment when the panel begins to open, bypassing its animation. By the way, if there are several panels, then all of them have the bug, except last one.

What are the steps to reproduce?

https://angular-p1bsmv.stackblitz.io/
https://stackblitz.com/edit/angular-p1bsmv

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

Incorrect behaviour. There was no such behaviour in the previous versions.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular Material 5.2.2
TypeScript 2.4.2

The bug was reproduced in the last versions of Chrome, FF, Edge, Opera.

Is there anything else we should know?

[angular-automatic-lock-bot]

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