Let widgets/clabel better handle overlapping axes.

In general, using event.inaxes/event.xdata/event.ydata can be
error-prone when there are overlapping axes, because only the topmost
axes is the "in"-axes.

In the specific case of widgets/clabel, in particular, we can instead
check whether an event occurred over the axes and if so compute
xdata/ydata accordingly.  Doing so allows correct handling of widgets on
axes for which there's also a twin axes on top of it.

Changes in the tests (other than the change in test_span_selector, which
tests the functionality here) ensure that the synthetic button presses
indeed occur on top of the desired axes (previously, do_event would
directly set event.inaxes and we'd just believe that "as is").

Author: anntzer

lib/matplotlib/contour.py

@@ -68,7 +68,7 @@ def _contour_labeler_event_handler(cs, inline, inline_spacing, event):
     elif (is_button and event.button == MouseButton.LEFT
           # On macOS/gtk, some keys return None.
           or is_key and event.key is not None):
-        if event.inaxes == cs.axes:
+        if cs.axes.contains(event)[0]:
             cs.add_label_near(event.x, event.y, transform=False,
                               inline=inline, inline_spacing=inline_spacing)
             canvas.draw()

lib/matplotlib/tests/test_widgets.py

@@ -643,6 +643,13 @@ def test_span_selector(ax, orientation, onmove_callback, kwargs):
     if onmove_callback:
         kwargs['onmove_callback'] = onmove
 
+    # While at it, also test that span selectors work in the presence of twin axes on
+    # top of the axes that contain the selector.  Note that we need to unforce the axes
+    # aspect here, otherwise the twin axes forces the original axes' limits (to respect
+    # aspect=1) which makes some of the values below go out of bounds.
+    ax.set_aspect("auto")
+    tax = ax.twinx()
+
     tool = widgets.SpanSelector(ax, onselect, orientation, **kwargs)
     do_event(tool, 'press', xdata=100, ydata=100, button=1)
     # move outside of axis
@@ -925,7 +932,7 @@ def mean(vmin, vmax):
 
     # Change span selector and check that the line is drawn/updated after its
     # value was updated by the callback
-    press_data = [4, 2]
+    press_data = [4, 0]
     move_data = [5, 2]
     release_data = [5, 2]
     do_event(span, 'press', xdata=press_data[0], ydata=press_data[1], button=1)
@@ -1033,7 +1040,7 @@ def test_TextBox(ax, toolbar):
 
     assert submit_event.call_count == 2
 
-    do_event(tool, '_click')
+    do_event(tool, '_click', xdata=.5, ydata=.5)  # Ensure the click is in the axes.
     do_event(tool, '_keypress', key='+')
     do_event(tool, '_keypress', key='5')
 
@@ -1632,7 +1639,8 @@ def test_polygon_selector_verts_setter(fig_test, fig_ref, draw_bounding_box):
 
 
 def test_polygon_selector_box(ax):
-    # Create a diamond shape
+    # Create a diamond (adjusting axes lims s.t. the diamond lies within axes limits).
+    ax.set(xlim=(-10, 50), ylim=(-10, 50))
     verts = [(20, 0), (0, 20), (20, 40), (40, 20)]
     event_sequence = [
         *polygon_place_vertex(*verts[0]),