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[Bug]: macOS backend reports wrong button for button_release_event #31899
Description
Bug summary
Run the attached code with the qtagg as well as the macosx backends.
While the mouse cursor is over the figure, perform the following:
- Click mouse. Release mouse
- Hold Option: Click mouse. Release mouse.
- Hold Control: Click mouse. Release mouse.
Code for reproduction
import matplotlib.pyplot as plt
def on_button_press(event):
print(f'button press: {event.button}')
def on_button_release(event):
print(f'button release: {event.button}')
fig = plt.figure()
fig.canvas.mpl_connect('button_press_event', on_button_press)
fig.canvas.mpl_connect('button_release_event', on_button_release)
plt.show()Actual outcome
qtagg:
button press: 1
button release: 1
button press: 1
button release: 1
button press: 3
button release: 3
macosx:
button press: 1
button release: 1
button press: 2
button release: 1
button press: 3
button release: 1
Expected outcome
Based on these code comments in _macosx.m:
if (modifier & NSEventModifierFlagControl)
/* emulate a right-button click */
button = 3;
else if (modifier & NSEventModifierFlagOption)
/* emulate a middle-button click */
button = 2;I would expect:
button press: 1
button release: 1
button press: 2
button release: 2
button press: 3
button release: 3
However, if alt+click should not result in "middle click", I would expect macosx to match qtagg.
Additional information
No response
Operating system
macOS 14.8.5
Matplotlib Version
dev
Matplotlib Backend
qtagg, macosx
Python version
3.14.5
Jupyter version
No response
Installation
None
Activity
[Bug]: qtagg reports wrong button when Option is released before the …
iccir commentedon Jul 15, 2026
I have tried to investigate the history of "alt+click" emulating a middle click. While control+click is a standard shortcut on macOS for a right click, alt+click has no system-default meaning.
As far as I can tell, this probably originated with Apple's XQuartz X11 server. There is a "enable_fake_buttons" default that can be enabled. By default, it is off. It is unclear if "Button 2" and "Button 3" as mentioned in the man page correspond to 2=middle/3=right or 2=right/3=middle. Various open-source projects running on darwin map these differently.
As this behavior dates back to the original inclusion of
_macosx.min the project from 2008. My assumption is that the original author used the XQuartz or a similar environment with alt+click set to emulate a middle mouse button.If this is the case, I don't believe that this behavior should be propagated to other backends. In addition, I'm more inclined to have the macosx backend only map control+click to a right click (as qtagg does). I'm not sure if others have strong opinions.
Add _emulatedLeftMouseDownButton for matplotlib#31899