You should report this to the game developers also.
I had this same issue and I fixed it be editing "~/.config/unity3d/Colossal Order/Cities: Skylines/prefs"
Set the width to the proper resolution of one of your screens. For example, I have 2 screens 1920x1080. The game detected my screen as 3840x1080. I changed 3840 to 1920 in the prefs file and it worked. I made no other changes and the game started up fullscreen on one monitor with working menus.
@richbellamy Wow, it solved my issue here successfully! Thank you!
@richbellamy thank you very much
@richbellamy had the same problem, this fixed it. Thanks!
@richbellamy confirmed that this works.
Here's a one-liner which will work if you have two 1920x1080 or 1920x1200 screens.
sed -i'.bak' -e 's/3840/1920/g' ~/.config/unity3d/Colossal\ Order/Cities\:\ Skylines/prefs
Had the bug, that fix it... I was 10 minutes away from asking for a refund.
The way the game is made now, anybody using multiple monitors the first time they start the game break it forever, unless they find up on this page... I can't believe they didn't patch it yet.
Nothing extracted yet.
Cities: Skylines is a game that looks like I'm going to enjoy a lot, once I'm actually able to play it.
When I launch the game, it starts up in fullscreen mode (annoying, but at least it didn't try to change the screen resolution for me). However within the screen it was able to take over (always my right screen) it renders the window offset by half a screen and doesn't seem to obey (or visibly show) keyboard controls which might otherwise work around a menu that can't be easily clicked on with a mouse; let alone which is half hidden from view.
I would prefer if the game started in a windowed mode of reasonable default size and then let me adjust settings to my tastes.
The game also doesn't reliably respond to even killall -9 Cities.x64, resulting in...
I will file a second bug with a feature request related to improving the steam client to work around these types of issues.