The reason I had to turn off Displays Have Separate Spaces is that a Window COMP will not span across multiple monitors unless that is off. That happens on both my Macs at least, 10.14 and 12.0.
It seems I got lucky in my first test, because I went back and tried a bunch more configurations and then the a couple of bugs appeared.
Bug1 as you reported, seems to always happen on the built-in laptop monitor, but when the dropdown is on the external monitor it works fine. This seems to be a bug with the Window COMP inside the Widgets not displaying the dropdown overlay on top. I recommend checking the menu on both monitors.
Bug2 if you set the built-in monitor to a custom display scaling like “More Space”, then the mouse click coordinates no longer are correctly aligned, they click into the panel with an offset.
Both of these bugs we’ll look at fixing, but in the meantime I did find a solution that works. If you use Custom Parameters and a Parameter COMP for UI, these gadgets work fine in all cases.
Check out the edited file, it has the Parameter COMP method and also I duplicated the panels to show up on both screens as this makes it easier to confirm if it is not working on both screens or just 1 screen as in my case.
DualMonitorDropdown.toe (45.3 KB)