What is this blue half on my screen?
Thats a glitch in the split view feature.  

I would suggest holding down the full-screen button   in the upper-left corner of Chrome. And select a different Window to enter split screen with. Then simply get out of split screen my click the same button on either of the windows. This should reset the feature.

If that doesn't work try running sudo killall WindowsServer and enter your password. This is what controls the split view function (no citation, it just appeared this way through minimal testing). This will log you out but hopefully fix the issue.