What is this blue half on my screen?

In the midst of plugging in my monitor and three-finger swiping up to move Chrome to the second desktop I got this.

I couldn't access my top bar (with the time and battery) and if I clicked the side that is not blue the blue would switch to the other side.

Unplugged my monitor and it stayed on my laptop screen.

What did I do?

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.