Only noticed this at night - May happen during daytime...
Window Mode
Open your inventory and have lots of stuff spread over many lines with no empty spaces in the full lines
Move your mouse cursor from the right hand side of the game window toward the inventory window and as your cursor reaches or just touches the right edge of the inventory window the tooltip thingy opens up showing you that you are currently pointing at the item in the next row down but on the far left hand side of the inventory window. Its as if the inventory window thinks there is a hidden cell on its edge and thinks it contains whatever is in the next row's first cell.
Cheers