The Crop selection in Unstable Gimp behaves in an unusual and erratic
manner.  This seems mostly due to the mouse pointer crossing a window boundary.

I'm experiencing this on Windows Gimp; I haven't had a chance to test on
Linux yet.

- The selection area sometimes stays at 0x0 and the origin is moved, rather
than resizing the box. Moving toward the top-left then drags the selection.
In effect, instead of dragging the bottom-right corner as usual, the
pointer seems to have grabbed the top-left corner and is "pushing" the
bottom-right corner along with it.
Normally either corner can cross its opposite and select beyond, but when
this happens, you can only "push" the opposite corner, even if you re-grab
a different point.

- The Crop dialog often pops up directly over the area about to be cropped.
(This has always been the case; but while somewhat annoying, this did not
adversely affect cropping in Gimp 1.2. It does now...)

- When crossing a window boundary -- including the border of the dialog
over the image -- often (but not always) the selection will be reset with
the origin at that point.
Occasionally, the selection will drop into Move mode instead.
And sometimes, the selection will freeze, and ignore mouse movement
altogether; when this happens, you need to cross a boundary and hope the
behaviour resets, otherwise the selected crop is performed on mouse-up.