Yes installed as Adminitrator, yes clean installs, yes happens on 2017 and 2018.
DanRathbun, You nailed it!
Solved!
I was running an extended desktop on a larger monitor, and when the SU window is open on the larger screen, the menus do not drop, move SU back to my primary smaller screen, and it has the drop-down menus.
I tested this on a few different large monitors and it is a repeatable bug.
Though it also still happens intermittently. When I started working on a clean install today on the larger monitor I had no problems, and then just a little while ago, the menus stopped dropping down again.
Still unsure what the actual trigger of the bug is due to the intermitency.
-----Edit—
Ah, and now that I have the SU window not maximised I am seeing that the menu is opening, but far to the right of the window, so perhaps when it is maximized it is opening the menus off screen.
—Edit—
Without changing any screen scaling settings, they were set to 125% on my laptop (primary), and 100% for my Ultrawide secondary monitor. I went into my Nvidia 940mx control panel > Manage 3D settings > Program Settings (tab) and adjusted specific settings for SU, specifically that when running SU “Multi-display/mixed-GPU acceleration” was changed to “Multiple display performance mode”
The drop menus are still not scaling quite right, a little distortion, but at least they show up below the field, and not off screen.