Connect to a guest which doesn't have spice-vdagent, so it doesn't allow to use the mouse in the client mode (eg fresh installed Windows guest).
Mouse will be ungrabbed automatically upon clicking to the spice-gtk widget.
As Uri pointed out it is a regression since https://cgit.freedesktop.org/spice/spice-gtk/commit/?id=2cb74f4ac1fc5c3601cbc780f7eefa6e867ba4d2
Seems that it is related to gtk version.
I can reproduce it with gtk 3.20 (latest Fedora), but I cannot reproduce it with gtk 3.14
Mostlikely is it due to recent changes - removed window from gtkstack and also from spice display widget
Also spice-widget uses gdk_pointer_grab() which is deprecated and gdk_device_grab / gdk_seat_grab should be used.
reverting gtk+ 9d0e8401caa937c4399dbe6d2aaa6dc4fe0254eb seem to solve it
Author: Marc-André Lureau <firstname.lastname@example.org>
Date: Tue May 3 19:32:31 2016 +0200
gtk: fix server-mode mouse