IDE: Improve popover retention heuristic

[matthew-carroll]

Currently we display a popover when hovering over a language construct. What constitutes a language construct, and what's displayed in the popover, is determined by the LSP.

The current popover is displayed above the text span. Any mouse movement away from the text span causes the popover to disappear. Immediately closing the popover prevents us from ever having a scroll experience in the popover, or providing any buttons in the popover.

We should create a heuristic to make it reasonably possible to move the cursor over the popover and interact with it.

Here's a video of some experimentation in Android Studio:

demo_android-studio-popovers.mp4

It's not entirely clear what the heuristic is in Android Studio, but here are some things that seem like they're happening:

• Mouse over code span keeps popover open
• Mouse over popover keeps popover open
• Moving the cursor out of the code span, but moving towards the popover, keeps the popover open
• Moving the cursor out of the code span, and moving away from the popover, closes the popover
• There's some kind of slop region, or slop distance which seems to prevent immediate closing of the popover even when moving away