lightdm greeter not shown properly on zesty

I have a laptop with a 3200x1800 screen, and zesty shows the lightdm greeter incorrectly. Most of the time it's only a square box on top-left corner and the rest is blank. Sometimes it shows the full screen but it's only the top-left quarter of a full desktop as it thinks the monitor is bigger.

On yakkety I had this:
[+2,58s] DEBUG: main-window.vala:187: Screen is 3200x1800 pixels
[+2,58s] DEBUG: main-window.vala:195: Monitor 0 is 3200x1800 pixels at 0,0
[+2,58s] DEBUG: unity-greeter.vala:608: Showing greeter
[+2,58s] DEBUG: unity-greeter.vala:244: Showing main window
[+2,89s] DEBUG: Read 8 bytes from daemon
[+2,89s] DEBUG: Read 38 bytes from daemon
[+2,89s] DEBUG: Prompt user with 1 message(s)
[+2,90s] DEBUG: _fcitx_connection_create_ic
[+2,90s] DEBUG: _fcitx_connection_watch
[+2,91s] DEBUG: background.vala:68: Making background /usr/share/backgrounds/gnome/Waterfalls.jpg at 3200x1800
[+2,92s] DEBUG: main-window.vala:187: Screen is 1600x900 pixels
[+2,92s] DEBUG: main-window.vala:195: Monitor 0 is 1600x900 pixels at 0,0

but in zesty this:
[+2,95s] DEBUG: main-window.vala:187: Screen is 3200x1800 pixels
[+2,95s] DEBUG: main-window.vala:195: Monitor 0 is 3200x1800 pixels at 0,0
[+2,95s] DEBUG: unity-greeter.vala:608: Showing greeter
[+2,95s] DEBUG: unity-greeter.vala:244: Showing main window
[+3,23s] DEBUG: Read 8 bytes from daemon
[+3,23s] DEBUG: Read 38 bytes from daemon
[+3,23s] DEBUG: Prompt user with 1 message(s)
[+3,24s] DEBUG: _fcitx_connection_create_ic
[+3,24s] DEBUG: _fcitx_connection_watch
[+3,25s] DEBUG: background.vala:68: Making background /usr/share/backgrounds/gnome/Waterfalls.jpg at 3200x1800
[+3,25s] DEBUG: main-window.vala:187: Screen is 1600x900 pixels
[+3,25s] DEBUG: main-window.vala:195: Monitor 0 is 3200x1800 pixels at 0,0

anyway, downgrading gtk fixes the bug for me.