To expand on the reason why I opened this issue: In kwin (and often X11 itself) we don't get the information if a window is a child window ("transient-for") and to which window this child window is attached, so to recognize which windows should be attached to other windows we rely on the window type.
So currently in xrdesktop the SteamVR menu will appear at a position determined by some initial window placement rules, and will not be attached to the vrmonitor window.
Nothing extracted yet.
I'm talking about this menu here
It has window type 0
The menus of the Steam client have type 4: