[EuiPopover] Constrain popover dimensions to its container

[chandlerprall]

Need to start with an investigation to better understand how the current sizing works (or doesn't), but a popover should limit its sizing to stay within its configured container (window by default).

After poking a little more into the guts of popover_positioning and EuiPopover, this is semi-interesting shenanigans as the positioning service doesn't actually ever set width on anything - it's deriving its calculations purely from the existing width of the panel content, so basically CSS itself is doing a confused psyduck once you set a % width on content popovers.

So, as Chandler mentioned in the above thread, we may need to start modifying EuiPopover to manually set width/height styles where we're currently not... but that may also cause other unexpected shenanigans with popover down the road. Fun times!

Originally posted by @constancecchen in #6146 (comment)

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