This is a feature I miss from Unity. They had a dock on the side that was usable with multiple monitors thanks to the sticky edge feature. Basically you had to apply pressure to move your mouse beyond the dock to the second monitor. 

This feature ensured that that you could move your mouse all the way to the edge (fitts law) to engage the dock, even if there was as second monitor connected.

On KDE, you have to aim carefully for the side side, otherwise your mouse will simply travel to the second screen.

Some people found this annoying in Unity and complained but I think it's a net positive overall. Most people are not constantly moving their mouse between monitors - one monitor is for information while the other one is where most of the mouse work happens.