Code embeds can get stuck up

[microbit-matt-hillsdon]

We've seen this happen from time to time but it can be hard to reproduce.

The best repro is to use the mouse to try to slice off a tiny corner of the code embeds in a reasonably swift movement. I can do it 9/10 this way whereas just moving over the code embeds it happens more like 1/20.

[microbit-matt-hillsdon]

What we know so far:

• It's unrelated to the suspicious looking delay in CodeEmbed.
• Possible sequence of events:
  • Mouse enter code embed
  • Set state to show pop-up
  • Mouse leave code embed
  • Re-render shows pop-up

You can't unconditionally lower the pop-up in the mouse leave handler as it also fires when the pop-up is raised which would immediately close it!

But we can lower the pop-up if we're not over either code area (and the have the same position).

[microbit-matt-hillsdon]

Fix in eae15a6885ca4960794b0eebd713910d976bf046.

I've left this in the less-reference branch, but if that gets reverted for any reason we'll want to keep this.

[microbit-matt-hillsdon]

Will close this when less-references is merged.