Hint: Normal Window doesn't work in master
GIMP version: git master
Operating System: Linux, Openbox window manager
In past versions of GIMP including 2.10, setting the Preference: Window Management: Window Manager Hints: Hint for docks and toolbox: Normal Window made it possible to have the image window above the toolbox and docks. In master, setting this preference makes no difference: the image window is hidden underneath the toolbox and docks regardless of which of the three settings you use, so the preference is doing nothing. (And yes, I've quit and restarted.)
This makes GIMP much harder to use on small screens, because you can't see a lot of the image window unless you make it tiny.
The window properties for image windows are similar in 2.10 vs. master, but in master, the toolbox has: WM_TRANSIENT_FOR(WINDOW): window id # 0x1e00056 which it did not have in 2.10. I'm guessing that's the difference. I'll attach the xprop files in case anyone wants to look for other difference.toolbox-master.txtimagewin-master.txttoolbox-2.10.txtimagewin-2.10.txt