Mark Window as damaged when decoration or shadow changes

Enforms thumbnails, screencasts and effects of these changes and
that the window needs repainting.
BUG:464417
FIXED-IN:5.27.1

src/scene/windowitem.cpp

@@ -16,6 +16,8 @@
 #include "window.h"
 #include "workspace.h"
 
+#include <KDecoration2/Decoration>
+
 namespace KWin
 {
 
@@ -219,6 +221,7 @@ void WindowItem::updateShadowItem()
         } else if (m_surfaceItem) {
             m_shadowItem->stackBefore(m_surfaceItem.get());
         }
+        markDamaged();
     } else {
         m_shadowItem.reset();
     }
@@ -236,6 +239,8 @@ void WindowItem::updateDecorationItem()
         } else if (m_surfaceItem) {
             m_decorationItem->stackBefore(m_surfaceItem.get());
         }
+        connect(m_window->decoration(), &KDecoration2::Decoration::damaged, this, &WindowItem::markDamaged);
+        markDamaged();
     } else {
         m_decorationItem.reset();
     }