Aside from the difference in highlighting, another difference I failed
to notice before is that in fundamental-mode the string "TODO" is
displayed with DejaVu Sans but in lisp-interaction-mode with DejaVu Sans
Mono, although I used the same Lisp code with the face property
(:inherit variable-pitch) to enter the string in both buffers.  I guess
lisp-interaction-mode inhibits variable-pitch face and that's the reason
the mouse-highlighting appears on the problematic characters in that
mode, in contrast to fundamental-mode.

Steve Berman