- move it closer to updateFocus / componentDidMount for easier context between the other two methods

- move to separate method
- create instance var

- specify `initialPopover` and add `transitionType` check, as the popover doesn't re-initialize when moving between panels in the same popover, and we don't want this to re-fire unnecessarily
- add E2E tests for popover behavior

- this makes it so EuiContextMenu doesn't call `.focus()` too early and hijack the `returnFocus` element that our focus trap dependency sets via `document.activeElement` - see elastic#5760 (comment)

+ Add Cypress E2E tests for popover close focus return (w/ bonus `initialFocus` regression test)

…t correctly return focus :(

- this is because of `shouldComponentUpdate` - the `items` API updates focus less than `children`, so `children` is still updating/hijacking focus after the popover focus trap returns focus to the button

- replace `updateFocus` with `takeInitialFocus`, and do not continue to update/hijack focus once initial focus has been set

 - this removes the need to restrict how often `EuiContextMenuPanel` updates (which also requires a bunch of tedious `items` diffing that we will no longer need)

…ethod

- it shouldn't be tied to the focus call anymore since the focus call no longer occurs after update, and makes more sense as a separate call

+ updates to logic:
  - do not run `tabbable` on `children` API since it won't even use the navigation - return early
  - use `this.backButton` instead for back button focusing, since `children` will no longer have `menuItems`
  - Check for a valid focusedItem - it's possible for consumers to either pass in bad indices or for `items` to update and the index to no longer exist

- Add E2E tests confirming changes & new logic work as expected

- rename `incrementFocusedItemIndex` to `focusMenuItem` and change args to be a bit more human readable

- instead of having the previous `updateFocus` handle up/down nav, we can simply call `.focus()` from within this method, and arrow navigation works as before

- note `?.focus();` - this is important to keep as users can start mashing up/down before `tabbable` is done running and there are any menu items to focus

- no specific E2E tests for this, tests should simply not be failing