diff --git a/libkwineffects/kwinglutils.h b/libkwineffects/kwinglutils.h
--- a/libkwineffects/kwinglutils.h
+++ b/libkwineffects/kwinglutils.h
@@ -534,6 +534,7 @@
     static QSize s_virtualScreenSize;
     static QRect s_virtualScreenGeometry;
     static qreal s_virtualScreenScale;
+    static GLint s_virtualScreenViewport[4];
 
     GLTexture mTexture;
     bool mValid;
diff --git a/libkwineffects/kwinglutils.cpp b/libkwineffects/kwinglutils.cpp
--- a/libkwineffects/kwinglutils.cpp
+++ b/libkwineffects/kwinglutils.cpp
@@ -1038,6 +1038,7 @@
 QSize GLRenderTarget::s_virtualScreenSize;
 QRect GLRenderTarget::s_virtualScreenGeometry;
 qreal GLRenderTarget::s_virtualScreenScale = 1.0;
+GLint GLRenderTarget::s_virtualScreenViewport[4];
 
 void GLRenderTarget::initStatic()
 {
@@ -1074,6 +1075,9 @@
 
 void GLRenderTarget::pushRenderTarget(GLRenderTarget* target)
 {
+    if (s_renderTargets.isEmpty()) {
+        glGetIntegerv(GL_VIEWPORT, s_virtualScreenViewport);
+    }
     target->enable();
     s_renderTargets.push(target);
 }
@@ -1086,10 +1090,8 @@
     if (!s_renderTargets.isEmpty()) {
         s_renderTargets.top()->enable();
     } else {
-        glViewport (-s_virtualScreenGeometry.x(), s_virtualScreenGeometry.height() - s_virtualScreenSize.height() - s_virtualScreenGeometry.y(),
-                           s_virtualScreenSize.width(), s_virtualScreenSize.height());
+        glViewport (s_virtualScreenViewport[0], s_virtualScreenViewport[1], s_virtualScreenViewport[2], s_virtualScreenViewport[3]);
     }
-
     return ret;
 }