Check mark appears next to "Auto-Hide" on the toolbar's popup menu, but it has no effect: the toolbar remains visible (I have it docked to a window edge).
It's working now, after I moved it to another edge and put it back to the previous edge. Go figure.
Not working again. Restarted my PC today and one docked toolbar on monitor 1 is auto-hiding but another docked toolbar on monitor 2 is not. I tried turning-off then turning-on "Auto-Hide", but that didn't work. So I tried moving the toolbar to another edge and back again, and that didn't work either. Then I moved the non-working toolbar from monitor 2 to monitor 1, and Auto-Hide works there. Monitor 1 is my main display in Windows display settings. Yesterday, Auto-Hide was working well on both monitors, but I didn't restart the PC all day.
I dragged the toolbar around to different edges and monitors and then back to monitor 2, again. It's working now. Something is definitely awry.
Floating toolbar docking and auto-hiding is actually managed by the Windows shell, the same part of Windows which manages the Taskbar, and you're probably seeing bugs in Windows itself.
The shell can get confused at times, but it usually snaps back to reality as soon as you move any docked toolbar (including the Taskbar itself, unless things have changed in newer versions of Windows). That makes it reevaluate all the other toolbars as well.
Not many programs use the docking functionality these days and (possibly as a result) Microsoft have somewhat neglected it. In Windows 10 is completely broken on systems which use two or more monitors set to different DPIs, for example. (Then again, a lot of things are broken in Windows 10 if you mix DPIs, and Windows' support for per-monitor DPI rather than system-wide DPI is still in its infancy.)
I thought that Windows toolbars were deprecated in the Windows API ("deskbands"?). I thought Opus was creating toolbars from scratch (i.e., not via Windows APIs). They aren't available, as far as I've been able to find, to the end user like they used to be (unfortunately). I'd rather you keep them in Opus even though they are erratic, rather than eliminate them for lack of desire to implement toolbars without the Windows API, for they mostly work ("knock on wood"). I have noted that they are "staying on top" now (that said after a couple of days of having "Auto-Hide" turned on), whereas in v11 they would not stay on top so much.