Inner letterbox jiggles as border is dragged

TBB 12.0.2 lists #32308 (closed) in its changelog. @richard closed that issue, saying, "Looks like we're done here." Huh? Didn't anyone test it?? Because I'm still observing jiggling/jitter in 12.0.2 Linux 64-bit.

1. In TBB 12.0.2 desktop, open https://arkenfox.github.io/TZP/tzp.html
2. Slowly drag one side of the window horizontally.
3. While dragging, observe the line for "inner window... [LB] [RFP NewWin]" flickering red and the numbers changing a few pixels.

Heck, you could try it on the page of the original issue, #32308 (closed), and watch the content wrapping go crazy.