Since bug 1117870, we wait for 10 unused vsync notifications before unobserving it, due to the preference gfx.vsync.compositor.unobserve-count set to 10 at https://searchfox.org/mozilla-central/rev/fac07284a9a996ddf968ea53adaf25c2a8b7c520/modules/libpref/init/StaticPrefList.yaml#5404-5407

The original code at https://searchfox.org/mozilla-esr45/rev/e58e94fb0696e9d99b8685348f02278a4bafced7/gfx/thebes/gfxPrefs.h#271-273 had this comment:

  // On b2g, in really bad cases, I've seen up to 80 ms delays between touch events and the main thread
  // processing them. So 80 ms / 16 = 5 vsync events. Double it up just to be on the safe side, so 10.

It seems this was designed for b2g, maybe we can now remove this code, or dramatically reduce the value of this preference?

Jeff, you might be familiar with this?