FIX: Make widget blitting compatible with swapped canvas #30591

timhoffm · 2025-09-20T23:58:56Z

First step towards improving blitting - towards #30503 and fixing #30575.

This PR proposes a minimal low-level API to store background information on the canvas.
It also adds higher-level methods save_blit_background() and load_blit_background() to AxesWidgets. Child classes can execrise managed background storage though this API.

I specifically would like to have feedback on the solution idea and API design. Note: This is not intended to be a solution for everything right away, but I would like to learn more on blitting through the case of widgets.

Open topics

Check whether widgets act reasonably if the background is pulled under them. - As implemented now, they won't crash, but we must ensure that they can recover the background. It may be (to be checked) that they just loose the background.
We still need to refactor useblit from statically including canvas blitting capability to dynamic handling.

Edit: Tested the following examples all with blitting enabled, and they work as expected (after closing the figure, plt.figure(fig); plt.show() recrates the figure and the widgets continue to work.

timhoffm · 2025-09-26T09:44:10Z

Recent update/commit: Support blitting with changing canvas in some widgets

Approach depends a bit on the widget, but generally:

self._useblit only stores the flag passed to __init__
canvas.supports_blit is checked dynamically when needed
we sometimes have the property sef.useblit, which is the effective
value of flag and capability
if animated artists are used, that state is set during
__init__ based on the flag. This relies on the fact that
the flag cannot be changed later, and that the value of
"animated" does not play a role for drawing canvases without blit support.

Supported:

Button
_SelectorWidget
SpanSelector
ToolLineHandles
ToolHandles
RectangleSelector
LassoSelector
PolygonSelector

Not supported yet:

CheckButtons
RadioButtons
Cursor
MultiCursor
Lasso

This should already fix #30575 as Selector widgets are covered.

Note: It's probably best to review the commits individually.

tacaswell · 2025-09-26T19:29:01Z

Drat, I did not re-look at this PR before I opened #30605

timhoffm · 2025-10-03T08:40:26Z

Rebased.

timhoffm · 2025-10-03T09:15:52Z

@tacaswell How do we want to continue here? I consider this PR complete for the handled widgets, and suggest that we review and merge it. The remaining widgets should be handled in a follow-up PR to keep this PR more manageable.

Here's a summary of this PR approach:

First commit: Create an infrastructure that safely handles background images by storing them on the canvas and defining an API in a way that there is no guarantee a background image is available. Consumers can try to obtain a background but must gracefully handle the case that no background is available.
This is the basis to be able to reasonably handle blit-backgrounds while swapping canvas. Apply this to widgets that store backgrounds.
Second commit: Fix dynamic changes in useblit on the individual Widgets case-by-case. Since logic slightly differs between widgets, we have to think them through individually. The list of currently fixed widgets is in FIX: Make widget blitting compatible with swapped canvas #30591 (comment).

lib/matplotlib/widgets.py

tacaswell · 2025-10-03T15:09:25Z

lib/matplotlib/widgets.py

        """Update the canvas cursor."""
        self.ax.get_figure(root=True).canvas.set_cursor(cursor)

+    def _save_blit_background(self, background):


Do we want to pass the bounding box in instead?

For symmetry reasons and to keep the changes within the PR minimal, I would leave the complete handling to the individual widgets for now.

They do

self._save_blit_background(self.canvas.copy_from_bbox(self.ax.bbox))

and

background = self._load_blit_background() if background is not None: self.canvas.restore_region(background)

It may be reasonable to refactor in a follow-up and move that logic to the base class.

lib/matplotlib/widgets.py

Approach depends a bit on the widget, but generally: - `self._useblit` only stores the flag passed to __init__ - canvas.supports_blit is checked dynamically when needed - we sometimes have the property `sef.useblit`, which is the effective value of flag and capability - if animated artists are needed that state is set during __init__ based on the flag. This relies on the fact that the flag cannot be changed later, and that the value of "animated" does not play a role for drawing canvases without blit support. Supported: - Button - _SelectorWidget - SpanSelector - ToolLineHandles - ToolHandles - RectangleSelector - LassoSelector - PolygonSelector Not supported yet: - CheckButtons - RadioButtons - Cursor - MultiCursor - Lasso

greglucas · 2025-12-18T20:37:24Z

lib/matplotlib/widgets.py


-        self._useblit = useblit and self.canvas.supports_blit
-        self._background = None
+        self._useblit = useblit and self.canvas.supports_blit  # TODO: make dynamic


@timhoffm are you planning on addressing this TODO in this PR or is it a follow-up?

This is a follow-up and can land in 3.11 or later.

QuLogic

This API is semi-private, so it's open to change a bit, but I think it looks good regardless.

QuLogic · 2025-12-18T20:34:59Z

lib/matplotlib/widgets.py


-        self._useblit = useblit and self.canvas.supports_blit
-        self._background = None
+        self._useblit = useblit and self.canvas.supports_blit  # TODO: make dynamic


Is this TODO necessary to complete before 3.11?

This is a follow-up and can land in 3.11 or later.

greglucas

I think this is good to go with the current implementation and the TODOs can be addressed in follow-up PRs, but will leave that up to Tim whether he wants to get it in now.

timhoffm · 2025-12-18T21:25:04Z

First and foremost. This covers the regression #30575. That's prerequisite for 3.11.

We can live with not supporting canvas swapping on all widgets for 3.11. So it's ok to leave the TODOs and #30503 open.

github-actions bot added topic: widgets/UI topic: canvas and figure manager labels Sep 20, 2025

timhoffm force-pushed the safe-blit branch 2 times, most recently from 890404a to 48346de Compare September 21, 2025 15:37

timhoffm mentioned this pull request Sep 26, 2025

Make Widget.useblit robust against underlying canvas changes #30503

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timhoffm force-pushed the safe-blit branch 2 times, most recently from b418b60 to f7ca7cc Compare September 26, 2025 10:21

timhoffm marked this pull request as ready for review September 26, 2025 10:46

github-actions bot added the status: needs rebase label Sep 30, 2025

timhoffm force-pushed the safe-blit branch from f7ca7cc to 80b08c2 Compare October 3, 2025 08:39

github-actions bot removed the status: needs rebase label Oct 3, 2025

timhoffm force-pushed the safe-blit branch from 80b08c2 to 47e1d44 Compare October 3, 2025 08:44

timhoffm mentioned this pull request Oct 3, 2025

MNT: Make useblit a property in Widget classes #30605

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tacaswell added this to the v3.11.0 milestone Oct 3, 2025

tacaswell added the Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions. label Oct 3, 2025

timhoffm force-pushed the safe-blit branch from 87fa6db to 747155e Compare October 3, 2025 18:28

timhoffm added 2 commits October 3, 2025 20:34

FIX: Make widget blitting compatible with swapped canvas

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timhoffm added the status: needs review label Oct 5, 2025

tacaswell linked an issue Oct 30, 2025 that may be closed by this pull request

[Bug]: Regression in widget behavior #30575

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