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GeezerVison is showing github.com — Notify slider observers before redrawing in set_val by huzaifarahman13 · Pull Request #32587 · matplotlib/matplotlib

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@huzaifarahman13

@huzaifarahman13 huzaifarahman13 commented Nov 10, 2026 •
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PR summary

Closes #32526

Slider.set_val and RangeSlider.set_val called draw_idle() before notifying the on_changed observers. Because of that, the first redraw showed stale state for any artists that were updated inside the callback.
This PR moves the redraw so it happens after the observers are notified, in both methods. Behaviour with drawon=False is unchanged.
I added test_slider_set_val_notifies_before_draw, which checks that the callback runs before the draw for both sliders, with drawon set to True and False. It fails on main and passes with this change.

AI Disclosure

I used an AI assistant to help locate the code and draft the change and test. I reviewed and ran everything locally and understand the change.

PR quality check

  • Use an expressive title
  • New and changed code is tested
  • [N/A] Plotting related features are demonstrated in an example
  • [N/A] New features and API changes have release notes
  • [N/A] Documentation complies with general and docstring guidelines

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Huzaifa Rahman
Notify slider observers before redrawing in set_val
75b0457
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@github-actions github-actions Bot added first-contribution topic: widgets/UI and removed first-contribution labels Nov 10, 2026
@huzaifarahman13 huzaifarahman13 mentioned this pull request Nov 10, 2026
[Bug]: Slider.set_val renders the figure before it notifies its observers, sometimes causing two renders per event #32526
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[Bug]: Slider.set_val renders the figure before it notifies its observers, sometimes causing two renders per event

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@huzaifarahman13
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