diff --git a/effects/taskbarthumbnail/taskbarthumbnail.cpp b/effects/taskbarthumbnail/taskbarthumbnail.cpp
index 9ec67542bf..934c59db9d 100644
--- a/effects/taskbarthumbnail/taskbarthumbnail.cpp
+++ b/effects/taskbarthumbnail/taskbarthumbnail.cpp
@@ -47,14 +47,6 @@ TaskbarThumbnailEffect::TaskbarThumbnailEffect()
     unsigned char dummy = 0;
     XChangeProperty( display(), rootWindow(), atom, atom, 8, PropModeReplace, &dummy, 1 );
 
-    // With the NVIDIA driver the alpha values are set to 0 when drawing a window texture
-    // without an alpha channel on an FBO that has an alpha channel.
-    // This creates problems when the FBO is drawn on the backbuffer with blending enabled,
-    // so for now we only do high quality scaling with windows with alpha channels with
-    // the NVIDIA driver.
-    const QByteArray vendor = (const char *) glGetString( GL_VENDOR );
-    alphaWindowsOnly = vendor.startsWith("NVIDIA");
-
     if ( GLShader::fragmentShaderSupported() &&
          GLShader::vertexShaderSupported() &&
          GLRenderTarget::supported() )
@@ -193,13 +185,13 @@ void TaskbarThumbnailEffect::paintWindow( EffectWindow* w, int mask, QRegion reg
             if( thumbw == NULL )
                 continue;
             WindowPaintData thumbData( thumbw );
-            thumbData.opacity = data.opacity;
+            thumbData.opacity *= data.opacity;
             QRect r;
 
 #ifdef KWIN_HAVE_OPENGL_COMPOSITING
             if( effects->compositingType() == KWin::OpenGLCompositing )
                 {
-                if ( shader && (!alphaWindowsOnly || thumbw->hasAlpha()) )
+                if ( shader )
                     {
                     int tx = thumb.rect.x() + w->x();
                     int ty = thumb.rect.y() + w->y();
diff --git a/effects/taskbarthumbnail/taskbarthumbnail.h b/effects/taskbarthumbnail/taskbarthumbnail.h
index 507e35285e..b62dac57d3 100644
--- a/effects/taskbarthumbnail/taskbarthumbnail.h
+++ b/effects/taskbarthumbnail/taskbarthumbnail.h
@@ -67,7 +67,6 @@ class TaskbarThumbnailEffect
         int uOffsets;
         int uKernel;
         int uKernelSize;
-        bool alphaWindowsOnly;
     };
 
 } // namespace
diff --git a/scene_opengl.cpp b/scene_opengl.cpp
index 2cfd18db0e..d75ffa6abc 100644
--- a/scene_opengl.cpp
+++ b/scene_opengl.cpp
@@ -1838,6 +1838,16 @@ void SceneOpenGL::Window::prepareRenderStates( TextureType type, double opacity,
             glTexEnvfv( GL_TEXTURE_ENV, GL_TEXTURE_ENV_COLOR, constant );
             }
         }
+    else if( !alpha && opaque )
+        {
+        float constant[] = { 1.0, 1.0, 1.0, 1.0 };
+        glTexEnvi( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE );
+        glTexEnvi( GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_REPLACE );
+        glTexEnvi( GL_TEXTURE_ENV, GL_SOURCE0_RGB, GL_TEXTURE );
+        glTexEnvi( GL_TEXTURE_ENV, GL_COMBINE_ALPHA, GL_REPLACE );
+        glTexEnvi( GL_TEXTURE_ENV, GL_SOURCE0_ALPHA, GL_CONSTANT );
+        glTexEnvfv( GL_TEXTURE_ENV, GL_TEXTURE_ENV_COLOR, constant );
+        }
     }
 
 void SceneOpenGL::Window::restoreStates( TextureType type, double opacity, double brightness, double saturation, GLShader* shader )
@@ -1893,9 +1903,9 @@ void SceneOpenGL::Window::restoreRenderStates( TextureType type, double opacity,
             glDisable( tex->target());
             glActiveTexture(GL_TEXTURE0);
             }
-        glTexEnvi( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE );
-        glColor4f( 0, 0, 0, 0 );
         }
+    glTexEnvi( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE );
+    glColor4f( 0, 0, 0, 0 );
 
     glPopAttrib();  // ENABLE_BIT
     }