Based ongit bisect it looks like#16028 caused a regression with interactive mode in Python 3.8, Qt5Agg backend (PyQt5=5.13.2), Linux x86_64 (at least; this is my system). The previous behavior (e.g., in98aceb1) is that just doing:

import matplotlib.pyplot as pltplt.ion()fig, ax = plt.subplots()

would pop up a plot, and:

• Typing in the terminal worked fine
• Hitting ctrl-C (and then enter) had normal terminal behavior:

  >>>Traceback (most recent call last):  File "<stdin>", line 1, in <module>KeyboardInterrupt>>>

But ine6ffa5d, I get:

• Typing is very slow in the terminal (worsens with plot complexity)
• Hitting ctrl-C kills the terminal:

  >>> Traceback (most recent call last):  File "/home/larsoner/python/matplotlib/lib/matplotlib/backends/backend_qt5.py", line 507, in _draw_idle    self.draw()...  File "/home/larsoner/python/matplotlib/lib/matplotlib/ticker.py", line 2081, in __call__    return self.tick_values(vmin, vmax)  File "/home/larsoner/python/matplotlib/lib/matplotlib/ticker.py", line 2089, in tick_values    locs = self._raw_ticks(vmin, vmax)  File "/home/larsoner/python/matplotlib/lib/matplotlib/ticker.py", line 2045, in _raw_ticks    istep = np.nonzero(steps >= raw_step)[0][0]KeyboardInterruptAborted

If this description is not clear I could record some screen-cast GIFs or something.