R1:fb9f3cc0b4ca

Restore the "buoyant" header in Differential

Summary:
Fixes T1591. This was removed long ago because it was a mess to implement and caused a bunch of weird issues, and also my tolerance for dealing with weird JS issues was much, much lower.

I have now survived the fires of JX.Scrollbar and would love to address 200 small nitpicks about obscure browser behaviors on Linux, so open the floodgates again.

A secondary goal here is to create room to add a global view state menu on the right, with 300 options…