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Fix: Optimize Cursor clearing on mouse exit to prevent lag #31426

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Merged
tacaswell merged 9 commits into from
Apr 20, 2026
Merged

Fix: Optimize Cursor clearing on mouse exit to prevent lag #31426

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94dc3df
Fix: Prevent Cursor from forcing full canvas draw when useblit=True (…
MohitPal2005 Mar 27, 2026
f2680d7
fix: use draw_idle to clear Cursor lag on move
MohitPal2005 Mar 31, 2026
649f67a
Safeguard blitting and add test coverage for Cursor onmove
MohitPal2005 Mar 31, 2026
59f00ab
Fix MockEvent attributes and formatting in test_widgets
MohitPal2005 Apr 1, 2026
9e6816f
Remove duplicate test function
MohitPal2005 Apr 1, 2026
6f454d2
Fix: restore draw() to fix cursor rendering
MohitPal2005 Apr 3, 2026
121d89f
Fix: Optimize Cursor onmove by conditionally blitting background
MohitPal2005 Apr 18, 2026
ce80b26
Fix: Clean up git merge artifacts and restore deleted tests
MohitPal2005 Apr 18, 2026
6b91973
Test: Restore accidentally removed overlapping axis in test
MohitPal2005 Apr 18, 2026
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38 changes: 25 additions & 13 deletions lib/matplotlib/tests/test_widgets.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1819,17 +1819,29 @@ def test_parent_axes_removal():
checks._clear(evt)


def test_cursor_overlapping_axes_blitting_warning():

@timhoffm timhoffm Apr 18, 2026

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Why did you remove this test?

"""Test that a warning is raised and useblit is disabled for overlapping axes."""
fig = plt.figure()
ax1 = fig.add_axes([0.1, 0.1, 0.8, 0.8])
ax2 = fig.add_axes([0.2, 0.2, 0.6, 0.6]) # Explicitly overlaps ax1

Comment on lines -1826 to -1827

@timhoffm timhoffm Apr 18, 2026

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Please don't change!

match_text = (
"Cursor blitting is currently not supported on "
"overlapping axes"
)
with pytest.warns(UserWarning, match=match_text):
cursor = widgets.Cursor(ax1, useblit=True)
def test_cursor_clear_on_move_outside():

@timhoffm timhoffm Apr 18, 2026

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I'm not clear that the test is very useful as is. It relies on and tests some internal state.

At the very minimum, do not hack into the event system, but use MouseEvent._process() to simulate mouse move and place the cursor inside or outside the Axes. See how other examples solve this.

"""Test that moving the mouse outside the axes clears the cursor safely."""
import matplotlib.pyplot as plt
from matplotlib.widgets import Cursor

assert cursor.useblit is False
fig, ax = plt.subplots()
cursor = Cursor(ax, useblit=True)

# Simulate the cursor lines being visible
cursor.linev.set_visible(True)
cursor.lineh.set_visible(True)
cursor.needclear = True

# Create a mock mouse event where 'inaxes' is None (meaning outside)
class MockEvent:
inaxes = None
x = 0
y = 0

# Trigger the movement cleanup
cursor.onmove(MockEvent())

# Verify the lines were hidden and the flag reset
assert not cursor.linev.get_visible()
assert not cursor.lineh.get_visible()
assert not cursor.needclear
13 changes: 11 additions & 2 deletions lib/matplotlib/widgets.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2119,7 +2119,16 @@ def onmove(self, event):
self.linev.set_visible(False)
self.lineh.set_visible(False)
if self.needclear:
self.canvas.draw()
if self.useblit:
background = self._load_blit_background()
if background is not None:
self.canvas.restore_region(background)
background = self._load_blit_background()
if self.useblit and background is not None:
self.canvas.restore_region(background)
self.canvas.blit(self.ax.bbox)
else:
self.canvas.draw()
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self.needclear = False
return
self.needclear = True
Expand All @@ -2138,7 +2147,7 @@ def onmove(self, event):
self.ax.draw_artist(self.lineh)
self.canvas.blit(self.ax.bbox)
else:
self.canvas.draw_idle()
self.canvas.draw()

@timhoffm timhoffm Apr 18, 2026

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Please explain why switching to draw() is the right thing to do.



class MultiCursor(Widget):
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