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

[anntzer]

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.

Closes #17476.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[QuLogic]

Is this only needed for RectangleSelector?

[QuLogic]

And if so, maybe this should override set_active instead?

[anntzer]

set_active calls update_background, so that's fine?
Haven't checked the other widgets.

[QuLogic]

EllipseSelector derives from RectangleSelector so it'll be fine; the others don't appear to be problematic.

[QuLogic]

Wait, I forgot to set useblit=True; this does affect PolygonSelector, which doesn't derive from RectangleSelector.

Also affects SpanSelector if you toggle it in the middle of selecting something.

[anntzer]

Do you have an example where this can easily be checked? Is that just a matter of moving the update_background implementation fully to the base class?

[QuLogic]

I essentially patched the examples to do what RectangleSelector did:

diff --git a/examples/widgets/polygon_selector_demo.py b/examples/widgets/polygon_selector_demo.py
index c1340679f3..68de0e0e9a 100644
--- a/examples/widgets/polygon_selector_demo.py
+++ b/examples/widgets/polygon_selector_demo.py
@@ -50,9 +50,17 @@ class SelectFromCollection:
         elif len(self.fc) == 1:
             self.fc = np.tile(self.fc, (self.Npts, 1))
 
-        self.poly = PolygonSelector(ax, self.onselect)
+        self.poly = PolygonSelector(ax, self.onselect, useblit=False)
         self.ind = []
 
+        self.cid = self.canvas.mpl_connect('key_press_event',
+                                           self.toggle_selector)
+
+    def toggle_selector(self, event):
+        if event.key == 't':
+            self.poly.set_active(not self.poly.active)
+            print('Selector toggled')
+
     def onselect(self, verts):
         path = Path(verts)
         self.ind = np.nonzero(path.contains_points(self.xys))[0]
diff --git a/examples/widgets/span_selector.py b/examples/widgets/span_selector.py
index 2e720f4c98..3955c8f540 100644
--- a/examples/widgets/span_selector.py
+++ b/examples/widgets/span_selector.py
@@ -46,9 +46,17 @@ def onselect(xmin, xmax):
 #
 
 
+def toggle_selector(event):
+    if event.key == 't':
+        span.set_active(not span.active)
+        print('Span toggled')
+
+
 span = SpanSelector(ax1, onselect, 'horizontal', useblit=True,
                     rectprops=dict(alpha=0.5, facecolor='red'))
 # Set useblit=True on most backends for enhanced performance.
 
+fig.canvas.mpl_connect('key_press_event', toggle_selector)
+
 
 plt.show()

[anntzer]

OK, moving everything to the base class seems to work.