use after idle after 0 incantation for TkTimer

richardsheridan · richardsheridan · commit ab3c6967fa6d · 2020-08-18T10:14:01.000-04:00
diff --git a/lib/matplotlib/backends/_backend_tk.py b/lib/matplotlib/backends/_backend_tk.py
@@ -96,7 +96,16 @@ def _on_timer(self):
         # if _timer is None, this means that _timer_stop has been called; so
         # don't recreate the timer in that case.
         if not self._single and self._timer:
-            self._timer = self.parent.after(self._interval, self._on_timer)
+            if self._interval:
+                self._timer = self.parent.after(self._interval, self._on_timer)
+            else:
+                # Edge case: Tcl after 0 *prepends* events to the queue
+                # so a 0 interval does not allow any other events to run.
+                # This incantation is cancellable and runs as fast as possible
+                # while also allowing events and drawing every frame. GH#18236
+                self._timer = self.parent.after_idle(
+                    lambda: self.parent.after(self._interval, self._on_timer)
+                )
         else:
             self._timer = None
 
