Account for all hover heights & prevent un-hover between popovers (#9257)

Fixes https://github.com/zed-industries/zed/issues/5227
Fixes https://github.com/zed-industries/zed/issues/7304

Release Notes:

- Fixed an issue where not all editor hover popovers would be accounted
for when choosing to hover above or below the mouse cursor
([#5227](https://github.com/zed-industries/zed/issues/5227)).
- Fixed an issue where editor hover could be dismissed by moving the
mouse between hover popovers
([#7304](https://github.com/zed-industries/zed/issues/7304)).

Co-authored-by: Antonio Scandurra <antonio@zed.dev>