restrict cross-vd activation to in-client distrib

QDialog or at least QMessageBox does this and I doubt Lubos' idea was to
"allow focus stealing if the client nags enough" - switching the VD is a
major interrupt and prevented when a new window shows up.
It should not be possible to stomp on ground and then receive
focus - notably not on the other desktop.

I assume the original idea was to let clients distribute focus *inside*
across virtual desktops, maybe also permit when no client was active,
but hardly otherwise.

BUG: 359683
FIXED-IN: 5.6
REVIEW: 127153

activation.cpp

@@ -607,6 +607,9 @@ bool Workspace::allowClientActivation(const KWin::AbstractClient *c, xcb_timesta
         return true;
     }
 
+    if (!c->isOnCurrentDesktop()) // we allowed explicit self-activation across virtual desktops
+        return false; // inside a client or if no client was active, but not otherwise
+
     // High FPS, not intr-client change. Only allow if the active client has only minor interest
     if (level > FSP::Medium && protection > FSP::Low)
         return false;