PR Summary

MouseEvent.key is just derived from the last key_press_event/key_release_event, so will be wrong e.g. if one presses a key while the canvas does not have focus, then move the mouse to the canvas and clicks.

In general I think it isn't possible to know what keys are pressed while a mouse even occurs; one can only have a list of modifiers (ctrl/alt/shift). There's a PR adding that at #6159 but it needs a bit of work, so let's document the limitation for now.

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