macosx: Clean up single-shot timers correctly

The `NSTimer` docs state that a non-repeating (aka single-shot in our
terms) timer is invalidated after it fires. This means that we should
not do it ourselves, and in fact it appears that the pointer itself is
no longer valid, so we would be passing an `invalidate` message to a
random object or segfault.

Author: QuLogic

lib/matplotlib/backends/backend_macosx.py

@@ -67,13 +67,9 @@ def draw_idle(self):
 
     def _single_shot_timer(self, callback):
         """Add a single shot timer with the given callback"""
-        # We need to explicitly stop and remove the timer after
-        # firing, otherwise segfaults will occur when trying to deallocate
-        # the singleshot timers.
         def callback_func(callback, timer):
             callback()
             self._timers.remove(timer)
-            timer.stop()
         timer = self.new_timer(interval=0)
         timer.single_shot = True
         timer.add_callback(callback_func, callback, timer)

src/_macosx.m

@@ -1741,6 +1741,11 @@ - (void)flagsChanged:(NSEvent *)event
                                          repeats: !single
                                            block: ^(NSTimer *timer) {
         gil_call_method((PyObject*)self, "_on_timer");
+        if (single) {
+            // A single-shot timer will be automatically invalidated when it fires, so
+            // we shouldn't do it ourselves when the object is deleted.
+            self->timer = NULL;
+        }
     }];
     // Schedule the timer on the main run loop which is needed
     // when updating the UI from a background thread