PR summary

Previously, the timers were dependent on the length of time it took for the timer callback to execute. This dispatches the callback to the main thread's task queue to avoid synchronously waiting on long-running callback tasks.

Note that I didn't add any tests here because there is inconsistency between backends, so other backends fail for these assertions currently. I've opened up an issue to discuss that: #29029

On main, this script prints every 2 seconds callback time + timer interval, with this PR it prints every second max(callback time, timer interval).

import time
import matplotlib.pyplot as plt

fig = plt.figure()

def on_timer():
    print(time.ctime())
    time.sleep(1)

timer = fig.canvas.new_timer(interval=1000)
timer.add_callback(on_timer)
timer.start()
fig.canvas.start_event_loop(10)

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines