I have an interesting problem for quite some time now. My menubar turns all sorts of states on external monitors.
I have a 3-monitor-setup on an iMac 5K. I use two external Dell displays to the left and right of the built-in screen. On my right display, when it is inactive, the menubar turns completely transparent, except for the status icons. On my left display, everything stays normal, whether active or not. On my main display the menubar turns opaque white (including status icons) when inactive.
Several reboots and even a macOS update (it happened on 10.13.0, too) didn't fix it, neither did manually killing SystemUIServer. What can I do?
Installed tweaks affecting the menubar (no difference when disabling): Bartender, BetterTouchTool
OS: macOS High Sierra 10.3.1 (17B1003)