The only way I know of diffing two open (in editor) files is to find their nodes
in the Project window, select them, and right-clicking Tools->Diff.  That is
VERY ornery if you have a couple large projects with deeply nested package
structures - especially since NB's Project window doesn't seem to track
currently open files.  I could have sworn I used to be able to do this diff'ing
by selecting one tab and then maybe right-clicking on the second file's tab and
selecting 'diff'....but maybe I'm hallucinating.  But consider this a suggestion
for improving the current implementation.

Here's my specific use case:
I'm looking for a specific class, so I do my usual Alt-Shift-O and I see two
classes with the same name but different projects/packages.  I peruse one...then
the other...they appear to be the same so I want to diff them.  Hm.....one is in
package a.b.c.d.e.f in Project A, the other is in g.h.j.k in Project B....gotta
find/select each: do a "Find in->Projects" on the first and, voila, it selects
in the project tree.  But after doing "Find in->Projects" for the second one,
the first one becomes unselected and one must search for that Class by reading
through the project tree and then right-clicking on it and Tools->Diff.....very
frustrating.