Regenerate background when RectangleSelector active-flag is set back on.

Otherwise, if the selector is made inactive and then back to active, the
"old" selector gets captured in the background image and spurious
appears.

No tests because good luck with that, but can be tried with
examples/widgets/rectangle_selector.py.

Author: anntzer

lib/matplotlib/widgets.py

@@ -1456,8 +1456,21 @@ def update_background(self, event):
         """Force an update of the background."""
         # If you add a call to `ignore` here, you'll want to check edge case:
         # `release` can call a draw event even when `ignore` is True.
-        if self.useblit:
+        if not self.useblit:
+            return
+        # Make sure that widget artists don't get accidentally included in the
+        # background, by re-rendering the background if needed (and then
+        # re-re-rendering the canvas with the visible widget artists).
+        needs_redraw = any(artist.get_visible() for artist in self.artists)
+        with ExitStack() as stack:
+            if needs_redraw:
+                for artist in self.artists:
+                    stack.callback(artist.set_visible, artist.get_visible())
+                    artist.set_visible(False)
+                self.canvas.draw()
             self.background = self.canvas.copy_from_bbox(self.ax.bbox)
+        if needs_redraw:
+            self.update()
 
     def connect_default_events(self):
         """Connect the major canvas events to methods."""