A merge removed every piece of an open line that had a merged region on
both sides. The end of a line that stops inside a shape has the same
region on both sides, so a click or a drag on the shape, or deleting it
in erase mode, cut the line back to where it enters the shape.
A piece is now removed only when it runs between two different merged
or deleted regions. A line that crosses a shape still splits it, and
merging the two halves still removes the piece between them.