1 files changed, 25 insertions, 1 deletions
diff --git a/src/widgets/kernel/qwidgetbackingstore.cpp b/src/widgets/kernel/qwidgetbackingstore.cpp
index d5c8691459..63419ea093 100644
--- a/src/widgets/kernel/qwidgetbackingstore.cpp
+++ b/src/widgets/kernel/qwidgetbackingstore.cpp
@@ -448,6 +448,26 @@ void QWidgetBackingStore::sendUpdateRequest(QWidget *widget, UpdateTime updateTi
     if (!widget)
         return;
 
+#ifndef QT_NO_OPENGL
+    // Having every repaint() leading to a sync/flush is bad as it causes
+    // compositing and waiting for vsync each and every time. Change to
+    // UpdateLater, except for approx. once per frame to prevent starvation in
+    // case the control does not get back to the event loop.
+    QWidget *w = widget->window();
+    if (updateTime == UpdateNow && w && w->windowHandle() && QWindowPrivate::get(w->windowHandle())->compositing) {
+        int refresh = 60;
+        QScreen *ws = w->windowHandle()->screen();
+        if (ws)
+            refresh = ws->refreshRate();
+        QWindowPrivate *wd = QWindowPrivate::get(w->windowHandle());
+        if (wd->lastComposeTime.isValid()) {
+            const qint64 elapsed = wd->lastComposeTime.elapsed();
+            if (elapsed <= qint64(1000.0f / refresh))
+                updateTime = UpdateLater;
+       }
+    }
+#endif
+
     switch (updateTime) {
     case UpdateLater:
         updateRequestSent = true;
@@ -621,7 +641,11 @@ void QWidgetBackingStore::markDirty(const QRect &rect, QWidget *widget,
 
 
     const QRect widgetRect = widget->d_func()->effectiveRectFor(rect);
-    const QRect translatedRect(widgetRect.translated(widget->mapTo(tlw, QPoint())));
+    QRect translatedRect = widgetRect;
+    if (widget != tlw)
+        translatedRect.translate(widget->mapTo(tlw, QPoint()));
+    // Graphics effects may exceed window size, clamp.
+    translatedRect = translatedRect.intersected(QRect(QPoint(), tlw->size()));
     if (qt_region_strictContains(dirty, translatedRect)) {
         if (updateTime == UpdateNow)
             sendUpdateRequest(tlw, updateTime);