["FIXED"] FREESYNC workaround: if you also have artifacts and the screen turns dark sometimes, you can resolve this by using 85Hz instead of 100Hz. It works perfectly for me. (Who has >85 FPS in games anyway? XD) For that you will need to set a custom resolution (it's not in the defaults of the screen).
On Linux:
xrandr --newmode "3440x1440_85.00" 611.25 3440 3720 4096 4752 1440 1443 1453 1514 -hsync +vsync
xrandr --addmode DisplayPort-0 3440x1440_85.00
xrandr --output DisplayPort-0 --mode 3440x1440_85.00
(Note: you need to change DisplayPort-0 to your own output)
On Windows:
Use your driver's control panel.