@MattE I was wondering the same thing…
I’ve just spent an hour looking at past WIP threads about the middle mouse button popup.
(honestly, I’ve found few to none comments about appreciating the new version… I might be biased, but not few comments about asking it to behave like in 7…)
RH-70733 is marked as “8.0 UI Freeze”.
And it mentions a possible “PopUpContainer” command that we could fit inside a macro (maybe?).
If I can argumentate a bit more, in Rhino opening a toolbar by holding a button already pops a “temporary container”. It would be just plenty to have that option for MMB (not to pops an already “anchored” permanent container).
I’m calling it “temporary” because it will disappear if you just use one of its buttons, but it will stick if the user drags it a bit:
(in this gif you can also notice me going nuts about “temporary container” having some unneeded extra space, and after draggin the container have even more unnneded extra space! You can’t fix the wasted space but it will also forget its size if you close it and reopen it! … 
… UI freeze soon?
Edit: this is actually important!
“temporary containers” and normal containers should always have the same size, like it is in 7. At least the MMB popup would strictly need to remember the wanted size/format. Many users have lots of buttons arranged for quick access by position memory.)
… anyway.
In Rhino 7 it was awesome!
We could have the popup toolbar pops as “temporary container” for a quick command, but dragging it a bit would stick it in place like a normal container! Excellent UX imho!
Bring it back please. Exactly like it was on Rhino 7.