Bug with UI target window

I'm having an issue with the UI that's been happening for a few months but I couldn't quite pin down what was causing it until today when I just couldnt stand it anymore. On left click with the mouse over turrets, minions, players, etc, the UI pops up with a target window on the top left corner. When this happens I get a visual bug that flashes a blue line on my screen in that corner and can also lock me out of using abilities or items. It doesn't happen 100% but it does happen quite often and I can no longer use spells or items until I hit escape which clears the target window and allows me to use them again. Any idea what's causing this or if there's a way to fix it besides just not using the target window at all? Thanks.