ExpansionPanelDetail content should not be focusable when ExpansionPanel is collapsed

[sarod]

Today when an ExpansionPanel is collapsed the content is still focusable by tabbing which leads to a poorly accessible UX.

• This is not a v0.x issue.
• I have searched the issues of this repository and believe that this is not a duplicate.

Expected Behavior 🤔

Content of ExpansionPanelDetail should not be focusable when the ExpansionPanel is collapsed.

Current Behavior 😯

Today when an ExpansionPanel is collapsed the content is still focusable by pressing keyboard "tab" key.

Examples 🌈

This can be reproduced on the ExpansionPanel demo https://codesandbox.io/s/l39qxq4pxz where tabbing focuses the panel content like chips and link and button.

Context 🔦

The current behavior does not promote good accessibility by default.
Managing focus could be done by custom code outside the library but that can become cumbersome. And most developer would probably not think of this or not have time to do it (The fact that it's not done in the demo page is a good example of that).

[eps1lon]

The current behavior does not promote good accessibility by default.

Fully agree. We fixed this already in #14465 which is in a working state since @material-ui/core@^4.0.0-alpha.2: https://codesandbox.io/s/3vy63w7kzp

[oliviertassinari]

Nice! So it's a duplicate of #12632.

[sarod]

Excellent news I'll close this one then. Thanks again for the tremendous work!