Works here.
What does xev report when you press Alt+Print ?

For Alt:
KeyPress event, serial 41, synthetic NO, window 0x4600001,
    root 0xb7, subw 0x0, time 129309161, (1031,711), root:(1091,763),
    state 0x10, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 41, synthetic NO, window 0x4600001,
    root 0xb7, subw 0x0, time 129309241, (1031,711), root:(1091,763),
    state 0x18, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

For print:
KeyPress event, serial 41, synthetic NO, window 0x4600001,
    root 0xb7, subw 0x4600002, time 129382030, (40,44), root:(731,466),
    state 0x10, keycode 107 (keysym 0xff61, Print), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 41, synthetic NO, window 0x4600001,
    root 0xb7, subw 0x4600002, time 129382124, (40,44), root:(731,466),
    state 0x10, keycode 107 (keysym 0xff61, Print), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

Xev does not register when I press to keys at once differently then when I press them after each other. I am on unity, I will try to use some kind of fallback (I hopte it is still installable) to see whether it is caused by that.

Yep, I had it installed without even knowing it. It does not work under GNOME Shell either.

I forgot to mention that you will have to 

gsettings set org.gnome.settings-daemon.plugins.media-keys active false

while testing with xev, to prevent key grabs from gnome-settings-daemon to interfere with the testing.

Ha, I am getting:

KeyPress event, serial 37, synthetic NO, window 0x3e00001,
    root 0xb7, subw 0x3e00002, time 150458023, (19,46), root:(21,157),
    state 0x18, keycode 107 (keysym 0xff15, Sys_Req), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 37, synthetic NO, window 0x3e00001,
    root 0xb7, subw 0x3e00002, time 150458023, (19,46), root:(21,157),
    state 0x18, keycode 107 (keysym 0xff15, Sys_Req), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False


Print Screen is SysRq on my keyboard, but only with the fn key (this is a t500, basically this one: http://media.photobucket.com/image/recent/dr_st/SL500/DSC02413.jpg )

SysRq can be returned for:
- Shift + PrintScreen
or
- Alt + PrintScreen

as seen in:
$ xmodmap -pke | grep Sys_Req
keycode 107 = Print Sys_Req Print Sys_Req

The code in gtk/gtkcellrendereraccel.c doesn't take that into account.

We're moving to gitlab! As part of this move, we are closing bugs that haven't seen activity in more than 5 years. If this issue is still imporant to you and
still relevant with GTK+ 3.22 or master, please consider creating a gitlab issue
for it.

Yes, it is still an issue, I just reproduced it.

The Keyboard Shortcuts in GNOME does not use GtkCellRendererAccel (anymore?), so if you reproduced that, it's not this bug

And all of Alt+Print, Ctrl+Print, and Shift-Print *do* work in GtkCellRendererAccel (e.g. using tests/testaccel.c), including special handling there for the Alt+Print case. This, of course, is under latest GTK+ 3.22

We need specific info on how to update this, where, using which widgets, in which versions of which programs, on which distro and compositor.

> And all of Alt+Print, Ctrl+Print, and Shift-Print *do* work in GtkCellRendererAccel

(and in the GNOME Control Centre's Keyboard Shortcuts too, though it doesn't use CellRendererAccel)

As announced a while ago, we are migrating to gitlab, and bugs that haven't seen activity in the last year or so will be not be migrated, but closed out in bugzilla.

If this bug is still relevant to you, you can open a new issue describing the symptoms and how to reproduce it with gtk 3.22.x or master in gitlab:

https://gitlab.gnome.org/GNOME/gtk/issues/new

After upgrading to Ubuntu 18.04, I can no longer reproduce it. I am on a different computer anyway.