Heh, ok, fair enough!
In e8b50d + / Xinerama support / + { + XmNenableXinerama, XmCEnableXinerama, + XmRBoolean, sizeof(Boolean), + Offset(display.enable_xinerama), + XmRImmediate, (XtPointer) False + }, Mixing tabs and spaces for indentation.
I don't think guvcview needs to or should change, I created a MR to libdecor to not attempt to use the gtk decoration plugin and fall back to the cairo one, if it's not run from the main thread. Cairo doesn't care what thread it is run from. Ideally libdecor can address this in the future to get GTK based decorations for this use case.