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set_val drew the canvas before processing the 'changed' callback, so a callback that updates other artists paid two renders per event and the first showed the previous data. Notify observers first, then draw once; a callback that draws itself still sees exactly one render of the final state.
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PR summary
Fixes #32526.
Slider.set_valandRangeSlider.set_valcalledcanvas.draw_idle()before processing the'changed'observers, so an app whose callback updates other artists paid two renders per event — and the first render showed the previous data. On the reporting app this halved the interactive frame rate (49 ms of a 97 ms event was the wasted render).This reorders both methods to notify observers first and draw once afterwards. A callback that calls
draw_idle()itself still gets exactly one render of the final state; a callback that doesn't draw keeps the widget's own draw. There is no case where the old ordering produced output the new ordering loses — the old first render was stale by construction.The repro from the issue now reports zero draws before the callback:
Both
SliderandRangeSliderare covered; the otherdraw_idlecall sites inwidgets.py(Cursor, MultiCursor) draw in event handlers, not before callbacks, and are untouched.AI Disclosure
The reproduction and diagnosis were assisted by an AI coding agent, verified independently by me (matching the issue reporter's workflow). The ordering change is two three-line moves reviewed against the observer contracts in
CallbackRegistry.PR quality check
Closes #32526.