[EuiPopover and EuiButtonGroup] Focus behavior when EuiButtonGroup is first element

[kertal]

When a form displayed in a EuiPopover is using a EuiButtonGroup as first element, it is not initially focused, even when using the initialFocus prop of the EuiPopover. So here's an example of our Discover implementation, the first focused element is the EuiSelect (when using initalFocus set to the first EuiButtonGroup, nothing is selected)

Here's the Kibana PR elastic/kibana#83399

[chandlerprall]

This is caused by the transition: all $euiAnimSpeedNormal ease-in-out declaration for .euiButtonGroup__buttons . Targeting background-color instead of all resolves, but I'm not sure what properties need to be included/watched.

Something about the transition: all flags the .euiButtonGroup__buttons div's computed visibility as hidden instead of visible during the popover's initial rendering, preventing its inputs from being focusable.

/cc @cchaos