Sometimes, the panel does not unhide when the cursor touches the (right, in this case) edge. When this happens, the cursor has always exactly 1px distance to the edge and it's impossible to move it to the edge. To workaround this, either press an assigned shortcut or move the cursor away from the edge and try again.
Works fine as far as I can tell from testing. Given this is 2 years old, I'm closing. I'm sure you'll poke me if it's still an issue :)