The following is described in terms of an activity diagram because it is most annoying there, although it also applies
to other diagram types.

Assume you want to resize a lifeline by dragging one of its little blue squares. You put the mouse on the blue square,
and the mouse pointer changes to that 45 degree double arrow indicating you may resize the thing. However, when you
click and drag, it is not the lifeline that reacts but some message line that happens to be nearby. (The same can happen
if you'd like to move a message line but the tool decides to select a lifeline. The same happens for combined fragments
-- in short, for everything that may overlap.) The result is a totally messed-up diagram. Combined with the lack of an
undo feature, this is a real showstopper.

This was the worst case: The tool displays one element as selected, but when you move or resize it, selects another one.
But even if the tool correctly displays the element it believes to be selected, there still is no way for the user to
enforce that another, overlapping element should be selected. This means clicking around again and again just to find a
point where the tool decides to select the right element.

So, there should be a way to select each each of a set of overlapping elements in a predictable way. First and foremost,
the element shown as selected by framing it in blue should reliably be the one that stays selected if clicked on.
Second, there should be some convention, like pressing Strg while clicking, to select each of those overlapping elements
in turn.