Disconnect manager when resizing figure for animation saving.

This is similar to what's done in savefig() to prevent the GUI window
from being resized, and is also helpful as this code may trigger after
the GUI window has been closed.

Author: anntzer

lib/matplotlib/animation.py

@@ -361,12 +361,10 @@ def finish(self):
     def grab_frame(self, **savefig_kwargs):
         # docstring inherited
         _log.debug('MovieWriter.grab_frame: Grabbing frame.')
-        # re-adjust the figure size in case it has been changed by the
-        # user.  We must ensure that every frame is the same size or
-        # the movie will not save correctly.
+        # Readjust the figure size in case it has been changed by the user.
+        # All frames must have the same size to save the movie correctly.
         self.fig.set_size_inches(self._w, self._h)
-        # Tell the figure to save its data to the sink, using the
-        # frame format and dpi.
+        # Save the figure data to the sink, using the frame format and dpi.
         self.fig.savefig(self._frame_sink(), format=self.frame_format,
                          dpi=self.dpi, **savefig_kwargs)
 
@@ -1113,10 +1111,12 @@ def func(current_frame: int, total_frames: int) -> Any
                       "frame size to vary, which is inappropriate for "
                       "animation.")
         # canvas._is_saving = True makes the draw_event animation-starting
-        # callback a no-op.
+        # callback a no-op; canvas.manager = None prevents resizing the GUI
+        # widget (both are likewise done in savefig()).
         with mpl.rc_context({'savefig.bbox': None}), \
              writer.saving(self._fig, filename, dpi), \
-             cbook._setattr_cm(self._fig.canvas, _is_saving=True):
+             cbook._setattr_cm(self._fig.canvas,
+                               _is_saving=True, manager=None):
             for anim in all_anim:
                 anim._init_draw()  # Clear the initial frame
             frame_number = 0