Drag a piece from the selection bar onto the level area, and position it partly off the edge of the level. The piece will be rejected and disappear entirely if the mouse coordinate is off the edge, i.e. the piece is more than halfway off, even though it's entirely reasonable to want to place pieces so that only a small part of them is on the level.
In version 1.9 I added some small gray borders, which allows you to drag the pieces slightly off the level area, but not too much. Before doing anything more, I would like to have some feedback whether this is enough or you need more.