The GdkDragContext should only listen to GDK_GRAB_BROKEN events sent to
its own pointer device. It turns out that the passive key grabs mistake
GDK into sending a GdkEventGrabBroken on the master keyboard, which the
DnD machinery mistakes as a signal to cancel the operation.
https://bugzilla.gnome.org/show_bug.cgi?id=766442