Test suggestion:
It would be useful to be able to identify skipped frames (thinking about
this recent thread).
In some of my work, the test I've used for this is to display a row of numbers, each number appearing for exactly one frame as we cycle through them. (All numbers have to be in a different visual position.)
Actually, after looking at the stopwatch test, that seems to be almost this, but I find it a lot less effective to try and read it because the numbers are always displayed, and the contrast between red and blue is not nearly as strong as if it would be if the numbers were appearing only on their frame. (Black would be better than that blue, and that dark blue is better than the red for contrast against white.) The little dots in the centre are only on for one frame, but they're small and red and in a circle instead of a row.
Additionally, a longer period than 10 may help; I've usually found 16 to be effective, in a row or a grid. 64 is probably too many, but 10 looks like too few to me, but I'm just "eyeballing" this, YMMV. A configurable period might be even better as it could help diagnose particular framerate mismatch problems too (e.g. 60 vs 50 fps).
Other versions of the 240p suite seem to have a "lag test" that is like your stopwatch but with bigger numbers arranged in a 4x2 grid; it seems that older versions had on/off numbers instead of red/blue, which I think was better, but given that the intent of that test was to be used with a camera, I don't think it made a difference for that purpose. My suggestion is for something that you should be able to see easily with the human eye, which wasn't really the original intent I think. A human test should try to emphasize and take advantage of persistence of vision (via greater contrast, or larger numbers, or only showing one at a time, etc.).