Sometimes, the mouse_release_event ending a pan/zoom can be lost, if it
occurs while the canvas does not have focus (a typical case is when a
context menu is implemented on top of the canvas, see example below);
this can result in rather confusing behavior as the pan/zoom continues
which no mouse button is pressed.  To fix this, always check that the
correct button is still pressed in the motion_notify_event handlers.

To test, use e.g.
```
from matplotlib import pyplot as plt
from matplotlib.backends.qt_compat import QtWidgets

def on_button_press(event):
    if event.button != 3:  # Right-click.
        return
    menu = QtWidgets.QMenu()
    menu.addAction("Some menu action", lambda: None)
    menu.exec(event.guiEvent.globalPosition().toPoint())

fig = plt.figure()
fig.canvas.mpl_connect("button_press_event", on_button_press)
fig.add_subplot()
plt.show()
```
enter pan/zoom mode, right-click to open the context menu, exit the
menu, and continue moving the mouse.