Prevent UI events from propagating beyond imgui

[panxinmiao]

After a UI event is handled by imgui, we generally don't want the event to be further processed by subsequent handlers (e.g., Pygfx).

For instance, in the following case, it is quite inconvenient

1.mp4

This PR allows imgui to capture canvas UI events and prevent them from further propagation, as shown below:

2.mp4

PS: The implementation might be a bit hacky, 😅 , and I’d be happy to hear your thoughts or suggestions.

[almarklein]

I think the general approach is good. I made a few comments.

We should also document this as part of the jupyter_rfb event spec, but I can do that later.

[kushalkolar]

Yay! Related: pygfx/pygfx#832

Nice that this PR seems to fix it in a general way

[panxinmiao]

Updated. :)

[Korijn]

• It's indeed a bit hacky to use this order argument and just fix the imgui renderer on -99... I wonder what a more general solution would look like. Is purely insertion order and the stop_propagation feature not enough?

If not, I'm fine with merging this.

[panxinmiao]

• It's indeed a bit hacky to use this order argument and just fix the imgui renderer on -99... I wonder what a more general solution would look like. Is purely insertion order and the stop_propagation feature not enough?

We need to provide a way to control the priority order of UI event propagation. For example, we would generally expect Imgui's event handler to be at the front of the queue. However, we cannot guarantee the order in which users initialize the ImguiRenderer and Pygfx Renderer, so we cannot control the insertion order in which the event handlers are added.

[almarklein]

Looking good. I can't think of a better way to control the order.

We can merge this before #573, so it ends up in the 0.17 release. Otherwise it will be part of a next release, which is also fine.