Keep hover annotation layers in sync with the mouse detector.

[gspencergoog]

Description

Adds a paint after detaching/attaching hover annotations to keep the annotation layers in sync with the annotations attached to the mouse detector.

Related Issues

Fixes #30744

Tests

Added a test that transfers focus between two listeners, and makes sure they both are detached at the end.

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Breaking Change

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• No, this is not a breaking change.

[dkwingsmt]

LGTM

[dkwingsmt]

L2560-2571 took me a while to understand, I think a clearer way is

if (newAnnotation == null && _hoverAnnotation == null)
  return
if (attached) {
  if (_hoverAnnotation != null) {
    RendererBinding.instance.mouseTracker.detachAnnotation(_hoverAnnotation);
  }
  if (newAnnotation != null) {
    RendererBinding.instance.mouseTracker.attachAnnotation(_hoverAnnotation);
  }
}
_hoverAnnotation = newAnnotation;

[gspencergoog]

That is clearer, except that I'd rather that _hoverAnnotation be up to date by the time attachAnnotation is called. I'll play with it some more and see if I can improve the readability.

[gspencergoog]

I basically reverted it to what it was before, since that was clearer. I added an assert which makes sure that something changed, since that was part of what I was trying to address with the reorg of the logic, but in reality it won't happen, since the callers all check and see that something changed before calling it.