The jump button is massive in the middle of the screen, and it gets in the way when trying to build playgrounds. E.g. when dragging to create a line of blocks, if you accidentally move the mouse over the jump button it treats it as if you've let go of the mouse. This is REALLY frustrating and happens literally all the time.

I think my students also find it odd that there's a jump button but not arrow key buttons. Given they need to use the keyboard to navigate anyway, there's no need for a jump button they have to click on - it's easier just to use the space bar.