This assumes that any empty selection of just one line that ends at the
start of a line is an after-end-of-line selection. We capture it on
mouseup to avoid comments appearing while dragging.
Known issues:
Occasionally, when the comment is created on an empty line, discarding
the comment can trigger another mouseup that creates a new comment.
Occasionally the selection of a line hangs around and double-clicking
after the end of the line doesn't do anything until the selection is cleared.
Change-Id: Ifeea4f25e2ee4d4643766e4bdff31a74fedca68f
51a9cb00e4