The exact definition of short shorts if difficult as a designer can work within any definition to make long shorts, or the wearer can wear his shorts to make them look longer or shorter as he pleases.
My 1980's Umbro football (soccer) shorts have 11" (278 mm) sides, and a 2" (50 mm) in-seam. Wearing them from the waist they are short, worn on the hip they appear 2" longer.
If you wanted to wear your shorts up to your armpits (see the 1940's swim team photos where the navel is covered by a very high cut suit), then with a small in-seam you would have short shorts that could be pulled down to the waist to produce strange long shorts.
In conclusion the definition must be based on the human body not on the fabric design, so when wearing short shorts the gluteus maximus should be covered, while most of the biceps femoris muscles should be exposed, see diagram on link:
http://www.flashmavi.com/weight_trai...le_system_legs
If you wear short shorts and find that your underwear is longer than your shorts, it is time to wear briefer underwear, or even Speedos as underwear. A couple of inches of boxer shorts hanging out of otherwise smart shiny shorts looks awful. Unless you need thermals, why are you wearing big underwear anyway?