This is part of the "snap" feature. I elected to turn it off entirely because I don't want it or need it.
The block is caused by the Windows 10 Aero Snap feature. It if you drag a window slowly, it will think you want to snap to the side/top border. You have 3 options that i know of:
1) Disable snapping in Display Settings / Multi-tasking. This disables ALL snapping, including the Windows 7 style snap-to-top-of-screen which you are probably use to. I tried this but I miss snapping to the top of the screen too much.
2) Move the mouse fast. This is your main option that actually works but is as annoying as hell, coz you'll often have to try again a couple of times when you forget to move the mouse fast.
3) Don't drag. use the WINDOWS ARROW key combination to snap windows around, or WINDOWS SHIFT ARROW to move between monitors.