Comment 33: With X11 clients, there is a (preconfigured, but changable) white list of clients whos grabs will be granted allowing them to work as intended. See org.gnome.mutter gsettings
Where is this white list?
Found the magic key in mutter and made a pull request: add 'xpra' to the list of xwayland clients that can do grabs
No reply so far..
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2613