[Collapse] Support flexbox

[ShaMan123]

Summary

I expect Collapse to be able to handle flexbox layout transition.

Examples

https://stackblitz.com/edit/github-nb4cly3i?file=src%2FApp.tsx

In the example there are 3 columns, the left one is collapsed.
Hit the Collapse/FlexCollapse button to toggle which component is used.
Hit the Open/Close button to see the transition in action.
Change the flex input.

Motivation

Using Collapse in a flexbox is not supported/behave unexpectedly

Search keywords: collapse, flex, flexbox

[ZeeshanTamboli]

Using Collapse in a flexbox is not supported/behave unexpectedly

What does not work? If I choose "Flex Collapse" button, and change the flex value to 2 in the TextField, it works.

And why do you need "Flex Collapse" component?

I don't understand what's wrong here..

[ShaMan123]

The point of the demo is to show that the regular Collapse component can't be used in a flexbox.
The Flex Collapse component is a custom component I created that I wish to be supported by mui as part of Collapse