Welcome to my simple but indispensable guide. This is how I fixed it:

• 1st. Go to Options -> Video and set your desired resolution. Set the game on windowed display mode.
  
• 2nd. Go to Options -> Control Options -> Smooth Mouse < Disabled >.
  
• 3rd. Make it borderless. Go to Steam->Library-> right click on Guns of Icarus Online -> Properties -> Launch options and type in "-popupwindow".

Make sure you also have it disabled on windows:
(Windows 7) Start -> Control Panel -> Hardware and Sound -> Mouse -> Pointer Options -> Enhance pointer prefission off