Use active() instead of surfaces().any in stack same_client_as

Setting the clipboard is only allowed when the keyboard focus is the
same client as the client trying to set the clipboard. We shouldn't
allow background windows in stacks to set the clipboard.

I don't know if anything else expects `same_client_as` to have this `any`
behavior.

src/shell/element/mod.rs

@@ -952,7 +952,7 @@ impl WaylandFocus for CosmicMapped {
     fn same_client_as(&self, object_id: &ObjectId) -> bool {
         match &self.element {
             CosmicMappedInternal::Window(w) => w.surface().same_client_as(object_id),
-            CosmicMappedInternal::Stack(s) => s.surfaces().any(|w| w.same_client_as(object_id)),
+            CosmicMappedInternal::Stack(s) => s.active().same_client_as(object_id),
             _ => false,
         }
     }