I find that frequently when trying to close a UI window in game (TP, inventory, hero-panel, etc.) I click the ‘X’ box in the upper right but it is so small that it doesn’t always close the window. I have to concentrate on making sure the mouse does not move outside the x-box before letting up on the mouse button.
My Suggestion
There is a lot of extra room around the button in the top of the UI window. Please consider enlarging the button to make it easier to click.