Hello,
I don't have a workaround, but possibly the same error, even without Global Menu widget.
I also use KDE, but without widgets or other extensions and 3rd party panels.
Only, the good old onboard functions of KDE.
However, since version 17.4.x the menu of the DaVinci Resolve window has disappeared.
(I have not yet found a way to make the menu reappear via the KDE window rules).
What my analyses have shown so far is that up to version 17.3.x everything works fine.
The menu bar is there, the program is fine to use.
From version 17.4.x (no matter which version from 17.4 or higher) the menu has disappeared. Downgrade and it's there again. Upgrade and it's gone.
I suspect that something about the window rendering in DaVinci Resolve was changed in version 17.4 and later. This seems to produce the menu-less window under Linux and (only?) KDE.
I hope that a Blackmagic employee will read this and possibly have an answer. Currently, I can only use up to version 17.3.2, everything above is flawed by the menu bug.
BTW, I use Manjaro Linux and DaVinci Resolve Studio.
Here you see the difference between the two versions:
