Ok so let's say I have two video clips with no handles. If I place those on the timeline manually and then drag a crossfade transition between them, PP complains about insufficient media. If I say do it anyway, PP duplicates frames and I see the little diagonal lines on the transition. Let's say I do the same thing with some still photo jpg files. In this case I do not get any complaints from PP and there are no diagonal lines. Is PP duplicating frames in this case, but not complaining or drawing diagonal lines because it knows the frames are all the same from a still image?
It is just the way that PPRo manages stills.
Titles are treated the same for example.
The diagonals are a warning more than anything else.