DOC: add api change note

ianhi · ianhi · commit d34d0fd840cb · 2020-08-04T19:03:49.000-04:00
diff --git a/doc/api/next_api_changes/behavior/18172-IHI.rst b/doc/api/next_api_changes/behavior/18172-IHI.rst
@@ -0,0 +1,6 @@
+webAgg backend no longer reports a middle click as a right click
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Previously when using the webAgg backend the event passed to a callback
+by `fig.canvas.mpl_connect('mouse_button_event', callback)` on a middle click
+would report `MouseButton.RIGHT` instead of `MouseButton.MIDDLE`
