1-frame hover delay on newly-spawned entities

[benfrankel]

Bevy version

0.16.0

What you did

Spawn a UI node directly under the user's cursor that changes its background color based on interaction state.

What went wrong

The frame that it spawns in, it has Interaction::None despite being under the user's cursor, so it flashes that color for 1 frame and then becomes Interaction::Hover the next frame.

Demonstration with colors changed for clarity:

2025-05-03.15-06-45.mp4

This seems to be because ui_focus_system iterates over UiStack to find hovered entities, but UiStack is not updated for the frame until ui_stack_system runs in PostUpdate.

[alice-i-cecile]

We should be able to move ui_focus_system, although this intersects with our "remove Interaction work".