framedropping + bypass = stuttering animation

framedropping + bypass = stuttering animation, similar to what happens if you give a non-bypass framedropping client only 2 buffers.

I've long suspected that because both:
  (a) framedropping requires two client buffers always available (instead of the usual one); and
  (b) bypass requires two compositor buffers always available (instead of the usual one);
that if you need both features at once then only four buffers will suffice to guarantee the compositor is never starved of new frames. Now for the first time this appears to be true.