Help with keyboard event capture and focus in/out

[Giovanni Panozzo <giovanni panozzo it>]

My application needs to capture keyboard events: when my windows is 
focused, I want all keystrokes routed to my app, and not to the WM.

For example pressing Super key on Ubutntu Unity, should not open the 
dash, but activate a function in my application.

So I used gdk_device_grab() to capture keyboard events.

But when gdk_device_grab() is in effect, I no longer receive 
"focus-out-event" signal, so my application can never release the grab.

How can I fix it ?

Attached is a full working example which exposes the problem, I'm using 
Ubuntu 16.04 (GTK 3.18).

Compile it with

gcc `pkg-config --cflags gtk+-3.0` -o testkbgrab \
   testkbgrab.c  `pkg-config --libs gtk+-3.0`

Thank you for your help.

Giovanni