Move keep-above clients to the Normal layer when showing desktop

Summary:
Implementation of the Show Desktop feature moves desktop windows to
the Above layer, but it doesn't take into account existing clients
that belong to the Above layer. If there are any, we have to move
them to a layer below (e.g. normal), otherwise those clients will be
visible when showing the desktop.

BUG: 406101

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D20153

abstract_client.cpp

@@ -374,7 +374,7 @@ Layer AbstractClient::belongsToLayer() const
     if (isActiveFullScreen())
         return ActiveLayer;
     if (keepAbove())
-        return AboveLayer;
+        return workspace()->showingDesktop() ? NormalLayer : AboveLayer;
 
     return NormalLayer;
 }

workspace.cpp

@@ -1276,7 +1276,9 @@ void Workspace::setShowingDesktop(bool showing)
     for (int i = stacking_order.count() - 1; i > -1; --i) {
         AbstractClient *c = qobject_cast<AbstractClient*>(stacking_order.at(i));
         if (c && c->isOnCurrentDesktop()) {
-            if (c->isDock()) {
+            // When showing desktop we'd like to raise docks above desktop
+            // windows and move keep-above clients to the normal layer.
+            if (c->isDock() || c->keepAbove()) {
                 c->updateLayer();
             } else if (c->isDesktop() && c->isShown(true)) {
                 c->updateLayer();