[EuiPopover] Initial isOpen works with outside click

[thompsongl]

Summary

Fixes #4460, in which initializing EuiPopover with isOpen=true would never set state.isOpenStable=true and therefore disable EuiOutsideClickDetector.

Previously, componentDidUpdate was the only place that state.isOpenStable was set to true. This moves the open popover logic outside of componentDidUpdate to a standalone function (onOpenPopover), which can now be called during componentDidMount.
The simpler way would just be set state.isOpenStable during componentDidMount, but that would bypass all timing functions. I could be persuaded that this simpler path is better, though.

Also, setting isOpening: true is no longer necessary in componentDidMount, but the Jest environment misses the update and causes tests and snapshots to break.

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile
- [ ] Checked in Chrome, Safari, Edge, and Firefox
- [ ] Props have proper autodocs and playground toggles
- [ ] Added documentation

• Checked Code Sandbox works for the any docs examples

- [ ] Added or updated jest tests

• Checked for breaking changes and labeled appropriately

- [ ] Checked for accessibility including keyboard-only and screenreader modes

• A changelog entry exists and is marked appropriately

[thompsongl]

jenkins test this

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4461/

[thompsongl]

jenkins test this

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4461/

[thompsongl]

Test failures are due to axe. The open popover prevents axe from traversing the page correctly. Will be resolved when I revert the 'revert me' docs commit.

[chandlerprall]

...x-state...

LGTM! Tested the modified example, could not get into a broken state. This is awesome, and this bug also affected the datagrid expansion popover - would you mind removing usages of renderPopoverOpen from data_grid_cell.tsx as well?

[thompsongl]

x-state

💯

would you mind removing usages of renderPopoverOpen from data_grid_cell.tsx as well?

Sure thing

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4461/