Just played around with "swisswatch" and regarded a strange behaviour,
it won't take focus for some actions.
So I xprop-ed it, to see, what there might be the reason.
Luckily, I had removed the decos, to have the clock displayed
purely on the screen.
Scrolled through Xprop's output, and what did I see ?
The "_NET_WM_STATE(ATOM)"-property, which I often use to check,
wether an app has set "_NET_WM_STATE_SKIP_PAGER" and such,
surprisingly shows "_OB_WM_STATE_UNDECORATED" !!!
The existing deco gets not marked there, maybe too obviously,
and I had never looked through Xprop for an undecorated app !
Like I Stated above:
Sometimes, you can't think stupid enough...
So this problem is also solved, finally !
Thank you all for your sympathy and help !