The general approach I think you would have to take is have a callback on motion_notify that starts a timer (and cancels a maybe running time) and then subscribe the actual callback to that timer.

I suggest starting with https://matplotlib.org/stable/api/backend_bases_api.html#matplotlib.backend_bases.FigureCanvasBase.new_timer to get timers integrated with the GUI toolkit and to make sure that the callbacks run on the main thread (threading.Timer will likely run into some "fun" thread related issues).

I don't see any technical reason why this can not be implemented outside of the core library, but I also suspect it will be subtle enough that it would be worth pulling it into core once it is working.

Is there prior art of APIs for hover that should be used as inspiration?

Going through matplotlib’s event mechanism feels relatively heavy. The underlying GUI frameworks should have appropriate events that we could subscribe to (e.g. QEvent::Tooltip for Qt). I’d expect that to be more efficient than restarting a timer through a Python function on every motion event.

See also #23378.

@albavilanova Could you describe the kind of annotations in your use case? Is it a simple text or a more complex annotation? Is it a constant annotation per Artist (Line, Rectangle…) or e.g. per data point / depending on the position within a rectangle, …)? Is the data constant and could be precomputed/provided via a high-level API such as plot([1, 2, 3], [1, 4, 9], tooltip=[“a”, “b”, “c”]) or does it need to be computed on-the fly via a function you provide?

It is simple text, generated using annotate, but the text isn't static as it is computed on-the-fly. We have given it a go with Tooltip from Qt and it seems to work much faster keeping motion_notify_event. Feel free to close the issue, thank you!

On reflection I agree with @timhoffm .

I'm going to close this as a duplicate of #23378.

Would it worth considering such an event ("non-motion" event) for widgets or selectors? This would a significant improvement for situation where the callback connected to the selectors are too slow to be live: for example instead of connecting the callback on motion_notify_event, it could be connect to this non-motion_notify_event.