Fix ExpansionPanel Performance Docs

[jameslaneconkling]

use mountOnEnter, not unmountOnExit, to prevent ExpansionPanel details from rendering before panel is expanded.

See this issue.

• I have followed (at least) the PR section of the contributing guide.

[mui-pr-bot]

No bundle size changes comparing 320b6f9...d4477c5

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[oliviertassinari]

Thanks for the concern but mountOnEnter was made on purpose to optimize for SEO. Hopefully, the description above the demo explains it. If you see something is not coherent, please let us know.

[jameslaneconkling]

@oliviertassinari This PR is just to change the docs on how to disable the SEO optimization, not on what the default behavior should be.

The current docs read:

---

Performance

The content of ExpansionPanels is mounted by default even if the panel is not expanded.
This default behavior has server-side rendering and SEO in mind.
If you render expensive component trees inside your panels or simply render many
panels it might be a good idea to change this default behavior by enabling the
unmountOnExit in TransitionProps:

<ExpansionPanel TransitionProps={{ unmountOnExit: true }} />

---

However, setting unmountOnExit will not prevent the details from mounting before expanding (as the docs suggest). Instead, mountOnEnter should be used to prevent eager rendering of content. The proposed edit to the docs for ExpansionPanel just reflects the proper way to disable the default behavior.

[oliviertassinari]

@jameslaneconkling I can't observe the behavior you are describing. Do you have a reproduction?

[jameslaneconkling]

@oliviertassinari damn it, can't reproduce. Must have been been doing something wrong on my end. Can confirm that TransitionProps={{ unmountOnExit: true }} is working as expected. Sorry for the confusion.

[oliviertassinari]

Thanks for the confirmation.