It is very difficult to select a combined fragments box (CFG) in the diagram
editor. Left-clicking a border of the CFG will not cause the CFG to be selected
if the pointer is within the bounding box for an object. (By the "bounding box",
I mean the box with the sizing handles.)

In other words, the precedence always favours the object, rather than the CFG.
The only way to select the CFG is to click on a CFG border that is in the gap
between the bounding boxes of two adjacent objects. (Of course, if the CFG is
outside any object, there is no problem.)

Here is the way that I think precedence works in other drafting packages.
Consider a short line in a rectangular box. The line does not touch the
rectangle at any point. Here is what happens when you click in several different
places.

1) Click outside the rectangle. Nothing is selected.
2) Click on a border of the rectangle. The rectangle is selected.
3) Click on the short line. The line is selected.
4) Click anywhere within the rectangle, but not on the line. The rectangle is
selected.

With some packages, you can use one of the control keys to allow for the fact
that the short line in our example may be exactly on the border of the
rectangle. If the short line is clicked, then either the line or the rectangle
is selected. If a control key is held down, and the short line is clicked, then
the alternative thing is selected. (I think it probably depends on which thing
is "in front".)