above Widget::GetCompositor() says:
// Returns the compositor for this Widget, note that this may change during
// the Widget's lifetime (e.g. when switching monitors on Chrome OS).
Is there any way for an external observer (an ordinary class that doesn't inherit from Widget or View) to know when a Widget's Compositor does change, aside from polling GetCompositor()? I may have missed something in the API when I checked; sorry if so.