After correctly applying the big fix on the guest too, the bug was still occurring but much more rarely.
But now, after I tried disabling Klipper (the KDE clipboard history manager) on both host and guest, the bug seems to have disappeared completely (and copy&paste still works fine, just without history).
I have yet to test what happens: if I keep Klipper disabled without any spice agent fix; and if I
keep Klipper disabled but with the small fix.
@paelzer I have an update.
After correctly applying the big fix on the guest too, the bug was still occurring but much more rarely.
But now, after I tried disabling Klipper (the KDE clipboard history manager) on both host and guest, the bug seems to have disappeared completely (and copy&paste still works fine, just without history).
I have yet to test what happens: if I keep Klipper disabled without any spice agent fix; and if I
keep Klipper disabled but with the small fix.