This is really strange. The inactive window title close button seems to be picking up the wrong bits on mouse over. As you can see the same glyph works when not hovered over. I thought it may be an overlapping button but I verified all close buttons are at the same offset. What's even more strange are the max and min buttons work on inactive. I'm sure you guys are using the same code. If you inspect it closely it's the right button size as if the image is shifted.