Painter widget handling events

[arthmis]

@cmyr proposed the idea of the painter widget handling events, in one of the zulip threads. Essentially he said

What i would do is add a new builder method to painter, with a name like... hm. paint_on_mouse_change? And that sets a bool in the painter that indicates it should invalidate itself during these events. A more robust way might be a builder fn, invalidate(mut self, fn: impl Fn(&Event) -> bool) -> Self? Basically you can pass it a closure that will receive an event and you can return a flag indicating whether you need to repaint. 🤔

I'm bringing this up in case it's still something up for consideration.

[cmyr]

To summarize: there's currently a limitation with the painter widget where it doesn't repaint when things like hot or active change, which make it not very useful if you're trying to paint the background for a custom button.

(The button example isn't the best because buttons request paint on hot/active change themselves, and so the painter does get repainted; but other widgets might not).

The rationale for this choice was that having it do this by default would cause a bunch of unnecessary paint calls if you didn't actually need it.

The current solution is to make sure some other widget in your hierarchy is handling this invalidation; if nothing is, you can use a custom controller or something.

My proposed solution is be to allow Painter to be optionally configured to do this itself; you can specify in some way that it should repaint on certain events (either by setting a repaint_on_mouse_change flag, or else by passing some closure Fn(&Event) -> bool that lets you specify exactly when you want to repaint).

This might be useful?

Would we also want this for focus change events?