With the 3.0.x branch, the window looks fine, with r24480 there is a black shadow. (with opengl off under virtualbox)
bug shown using deepin-music
Bisecting with r24285 re-added:
r23802 touches dozens of files...
Splitting r23802 into chunks:
Fixed in r24498.
(discovered a related issue on macos with this app: #2495)
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2493