[core] Improve a11y for Collapse, ExpansionPanel and Grow

[eps1lon]

• remove redundant aria-hidden, visibility is a better alternative
• add explicit boolean cast to aria-expanded in ExpansionPanelSummary. aria-expanded={undefined} will not appear in the DOM

Also in this PR: It's not vibility or visiblity. It's visibility.

[oliviertassinari]

What's the need for this change?

expanded: PropTypes.bool,

[eps1lon]

If it's passed as undefined it won't appear in the DOM not indicating that the element expands some content.

[oliviertassinari]

In practice, it should always be defined:
https://github.com/mui-org/material-ui/blob/64013d6c817b3c9bf2bd006aee2ac3f4a948a01f/packages/material-ui/src/ExpansionPanel/ExpansionPanel.js#L134-L137

However, we can't used .isRequired because it depends on the clone element method.

[eps1lon]

Are we arguing whether we should put this in? Why not be safe and always have it as a boolean. It's explicit, helps test. Could you elaborate what the downsides are here?

[oliviertassinari]

What I'm saying is that we can be "smart" about it, "smart" in the sense that the logic assumes more context. I'm showing an alternative viable option. I'm not saying it's better. We know that in practice, the Boolean() will be equivalent to an identity function, so it's not a requirement.

It's explicit, helps test.

I believe the tests could always provide a boolean value if we want to match reality. We could also add a warning in the render method.

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So now, what's best? I would go for reducing the entropy: add a runtime warning / throw if expanded is undefined. It's one less case to consider, no defensive checks etc. (It's not a really important matter, pick what you think is best)

[eps1lon]

You're right. I'm removing this. I think it might be a better idea to move those controlled props into context anyway. Gets rid of cloneElement, allows a fragment as a child and we can improve the DX by setting the default value of expanded to a boolean. That's for another PR though

[eps1lon]

Another codecov/codecov-bash#157: https://circleci.com/gh/mui-org/material-ui/73496

[oliviertassinari]

lol…