Test on synthetic events. Log your transitions. And for the love of all that is responsive, never, ever call register_pointer_focus inside a layout pass.
Registration is the act of notifying the system: “This window/view/widget is now the target. Direct all pointer events to it until further notice.” pointer focus registration code
If you’re building a new UI system today, to application code. Abstract it. Or suffer the consequences. Closing Thoughts Pointer focus registration is a 40-year-old problem that we still solve badly. Every modal dialog that steals focus while you’re typing? That’s a registration bug. Every click that falls into the void between scrolling frames? Also a registration bug. Test on synthetic events
By a recovering systems programmer
def register_pointer_focus(candidate, event): # 1. Pre-condition: candidate is alive and hittable assert candidate.is_alive() assert candidate.hit_test(event.x, event.y) == True # 2. Invalidate current focus without releasing events yet old_focus = system.pointer_focus system.pointer_focus = None Registration is the act of notifying the system:
occurs when pointer focus registers and releases multiple times within a single frame—usually due to overlapping transparent hitboxes or async layout recalculations.