Seems to be there for the sole purpose of ensuring the button
shall receive the key release on keyboard-triggered activation.

For the cases where this makes sense (eg. comboboxes, menubuttons,
...) gtk+ already does ensure the menu is popup after key release.
This makes the grab pretty useless, and there's many other cases
where it doesn't make sense (eg. button being activated
programmatically from an event handler in another widget).

Fixes button activation unintendedly triggering shortcut inhibition
on wayland.

https://bugzilla.gnome.org/show_bug.cgi?id=786480