Hello Yves. Thanks for your hints. I just wanted to express that XFCE4 is unusable in this special situation with SPICE. Bug reports should contain a line of what the software is expected to do. Perhaps I got the wording wrong as I'm no native speaker. I know FOSS is often maintained in free time and appreciate that.
Nevertheless I would classify this behaviour as "bug", not "wishlist". I will file a bug in the xfce bugtracker. If this would be fixed in XFCE4 upstream, will it get into buster as a fixup? Regards, Martin