fix flicker with fullscreen effects

BUG: 304375
FIXED-IN: 4.9.1
REVIEW: 106142

composite.cpp

@@ -463,14 +463,21 @@ void Compositor::timerEvent(QTimerEvent *te)
         QObject::timerEvent(te);
 }
 
-static bool s_pending = false;
+static int s_pendingFlushes = 0;
 void Compositor::performCompositing()
 {
     if (!isOverlayWindowVisible())
         return; // nothing is visible anyway
 
     bool pending = !repaints_region.isEmpty() || windowRepaintsPending();
-    if (!(pending || s_pending)) {
+    if (pending)
+        s_pendingFlushes = 3;
+    else if (m_scene->hasPendingFlush())
+        --s_pendingFlushes;
+    else
+        s_pendingFlushes = 0;
+    if (s_pendingFlushes < 1) {
+        s_pendingFlushes = 0;
         m_scene->idle();
         // Note: It would seem here we should undo suspended unredirect, but when scenes need
         // it for some reason, e.g. transformations or translucency, the next pass that does not
@@ -478,7 +485,7 @@ void Compositor::performCompositing()
         // Otherwise the window would not be painted normally anyway.
         return;
     }
-    s_pending = pending;
+
     // create a list of all windows in the stacking order
     ToplevelList windows = Workspace::self()->xStackingOrder();
     foreach (EffectWindow * c, static_cast< EffectsHandlerImpl* >(effects)->elevatedWindows()) {

scene.h

@@ -49,6 +49,9 @@ public:
     // Returns true if the ctor failed to properly initialize.
     virtual bool initFailed() const = 0;
     virtual CompositingType compositingType() const = 0;
+
+    virtual bool hasPendingFlush() const { return false; }
+
     // Repaints the given screen areas, windows provides the stacking order.
     // The entry point for the main part of the painting pass.
     // returns the time since the last vblank signal - if there's one

scene_opengl.h

@@ -46,6 +46,7 @@ public:
     virtual CompositingType compositingType() const {
         return OpenGLCompositing;
     }
+    virtual bool hasPendingFlush() const { return !m_lastDamage.isEmpty(); }
     virtual int paint(QRegion damage, ToplevelList windows);
     virtual void windowAdded(Toplevel*);
     virtual void windowDeleted(Deleted*);