diff --git a/shell_client.cpp b/shell_client.cpp
index 9c75487f7e..c9b3c94645 100644
--- a/shell_client.cpp
+++ b/shell_client.cpp
@@ -165,6 +165,7 @@ void ShellClient::addDamage(const QRegion &damage)
         }
         setGeometry(QRect(position, m_clientSize));
     }
+    setDepth(m_shellSurface->surface()->buffer()->hasAlphaChannel() ? 32 : 24);
     setReadyForPainting();
     Toplevel::addDamage(damage);
 }
diff --git a/toplevel.cpp b/toplevel.cpp
index 48724cbee4..8f80323e82 100644
--- a/toplevel.cpp
+++ b/toplevel.cpp
@@ -42,6 +42,7 @@ namespace KWin
 
 Toplevel::Toplevel()
     : m_visual(XCB_NONE)
+    , bit_depth(24)
     , info(NULL)
     , ready_for_painting(true)
     , m_isDamaged(false)
@@ -475,6 +476,18 @@ QByteArray Toplevel::windowRole() const
     return QByteArray(info->windowRole());
 }
 
+void Toplevel::setDepth(int depth)
+{
+    if (bit_depth == depth) {
+        return;
+    }
+    const bool oldAlpha = hasAlpha();
+    bit_depth = depth;
+    if (oldAlpha != hasAlpha()) {
+        emit hasAlphaChanged();
+    }
+}
+
 } // namespace
 
 #include "toplevel.moc"
diff --git a/toplevel.h b/toplevel.h
index ea21734546..3aea1bc0c3 100644
--- a/toplevel.h
+++ b/toplevel.h
@@ -71,7 +71,7 @@ class Toplevel
     : public QObject
 {
     Q_OBJECT
-    Q_PROPERTY(bool alpha READ hasAlpha CONSTANT)
+    Q_PROPERTY(bool alpha READ hasAlpha NOTIFY hasAlphaChanged)
     Q_PROPERTY(qulonglong frameId READ frameId)
     Q_PROPERTY(QRect geometry READ geometry NOTIFY geometryChanged)
     Q_PROPERTY(QRect visibleRect READ visibleRect)
@@ -414,6 +414,10 @@ Q_SIGNALS:
      * @since 5.3
      **/
     void surfaceIdChanged(quint32);
+    /**
+     * @since 5.4
+     **/
+    void hasAlphaChanged();
 
 protected Q_SLOTS:
     /**
@@ -460,6 +464,7 @@ protected:
     friend QDebug& operator<<(QDebug& stream, const Toplevel*);
     void deleteEffectWindow();
     virtual bool shouldUnredirect() const = 0;
+    void setDepth(int depth);
     QRect geom;
     xcb_visualid_t m_visual;
     int bit_depth;