Fixes #31899

PR summary

The issue reports on macOS that Option+click and Ctrl+click were not correctly mapped to middle and right button clicks, respectively.

In qtagg, Option+click was hardcoded to button = 1 as backend_qt.py did not contain a button modifier mapping. So event.button() returned the physical button, which resulted in Option+click incorrectly emulating button 1 instead of button 2.

In macosx, the mouseDown: method was correctly mapped, however, mouseUp: hardcoded button = 1, which resulted in press and release being inconsistent.

Fixes

For qtagg, I added a _remap_darwin_button(...) static method in backend_qt.py that maps Alt/Option+left-click to the middle button on macOS.

For macosx, I added a static variable effectiveLeftButton to store the resolved button at press time, allowing mouseUp: to read it back instead of hardcoding 1.

Minimum self-contained example

Instructions:

• Left Click
• Left Click + Option
• Left Click + Control

import matplotlib
matplotlib.use('macosx') # macosx/qtagg
import matplotlib.pyplot as plt

def on_press(event):
    print(f'button press:   {event.button}')

def on_release(event):
    print(f'button release: {event.button}')
    print()

fig = plt.figure()
fig.canvas.mpl_connect('button_press_event', on_press)
fig.canvas.mpl_connect('button_release_event', on_release)
plt.show()

Outputs

Current

macosx

button press:   1
button release: 1

button press:   2
button release: 1

button press:   3
button release: 1

qtagg (PyQt6)

button press:   1
button release: 1

button press:   1
button release: 1

button press:   3
button release: 3

With Fixes

macosx & qtagg

button press:   1
button release: 1

button press:   2
button release: 2

button press:   3
button release: 3

Question

As raised in the initial issue, should the intended behaviour for the action Alt + left-click result in the middle mouse? This implements it, but can be dropped by removing _remap_darwin_button(...) helper function.

AI Disclosure

Claude Opus 4.8 was utilised to proofread my changes.

PR checklist

• closes [Bug]: macOS backend reports wrong button for button_release_event #31899
• new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines