In windows mode, the window produced is the correct size according to windows, but has a 1 pixel border on the left, right, and bottom that is 1 pixel wide and allows for clicking on the desktop, causing the game to lose focus which disables skill usage.
In fullscreen mode, the opposite is the problem, allowing the game to expand 1 pixel beyond the screen into other monitors. If those other monitors don’t have the same exact size and refresh rate, the game will stutter terribly.
Suggested fix: Correct the border sizes.