qt: Mark canvas for re-draw after savefig

In some cases, Qt will itself trigger a paint event after closing the
file save dialog. When that happens, we need to be sure that the
internal canvas is re-drawn. However, if the user is using an
automatically-chosen Qt backend but saving with a different backend
(such as pgf), we do not want to trigger a full draw in Qt, so just set
the flag for next time.

Fixes matplotlib#26272

Author: QuLogic

lib/matplotlib/backends/backend_qtagg.py

@@ -71,6 +71,15 @@ def paintEvent(self, event):
         finally:
             painter.end()
 
+    def print_figure(self, *args, **kwargs):
+        super().print_figure(*args, **kwargs)
+        # In some cases, Qt will itself trigger a paint event after closing the file
+        # save dialog. When that happens, we need to be sure that the internal canvas is
+        # re-drawn. However, if the user is using an automatically-chosen Qt backend but
+        # saving with a different backend (such as pgf), we do not want to trigger a
+        # full draw in Qt, so just set the flag for next time.
+        self._draw_pending = True
+
 
 @_BackendQT.export
 class _BackendQTAgg(_BackendQT):