bug 362015. r=vlad. makes us use a region for drawing to avoid painting too much.

widget/src/cocoa/nsChildView.mm

@@ -2515,33 +2515,43 @@ NSEvent* globalDragEvent = nil;
 
   nsRefPtr<gfxContext> targetContext = new gfxContext(targetSurface);
 
+  nsCOMPtr<nsIRenderingContext> rc;
+  mGeckoChild->GetDeviceContext()->CreateRenderingContextInstance(*getter_AddRefs(rc));
+  rc->Init(mGeckoChild->GetDeviceContext(), targetContext);
+
+  /* clip and build a region */
+  nsCOMPtr<nsIRegion> rgn(do_CreateInstance(kRegionCID));
+  if (rgn)
+    rgn->Init();
+
   const NSRect *rects;
   int count, i;
   [self getRectsBeingDrawn:&rects count:&count];
   for (i = 0; i < count; ++i) {
-    targetContext->Rectangle(gfxRect(rects[i].origin.x, rects[i].origin.y,
-                                     rects[i].size.width, rects[i].size.height));
+    const NSRect& r = rects[i];
+
+    /* add to the region */
+    if (rgn)
+      rgn->Union(r.origin.x, r.origin.y, r.size.width, r.size.height);
+
+    /* to the context for clipping */
+    targetContext->Rectangle(gfxRect(r.origin.x, r.origin.y, r.size.width, r.size.height));
   }
   targetContext->Clip();
   
-  nsCOMPtr<nsIRenderingContext> rc;
-  mGeckoChild->GetDeviceContext()->CreateRenderingContextInstance(*getter_AddRefs(rc));
-  rc->Init (mGeckoChild->GetDeviceContext(), targetContext);
-  
-  nsRect r, tr;
-  NSRectToGeckoRect(aRect, r);
-  tr = r;
-
-  mGeckoChild->LocalToWindowCoordinate(tr);
-  //targetContext->Translate(gfxPoint(tr.x, tr.y));
+  /* bounding box of the dirty area */
+  nsRect fullRect;
+  NSRectToGeckoRect(aRect, fullRect);
 
   nsPaintEvent paintEvent(PR_TRUE, NS_PAINT, mGeckoChild);
   paintEvent.renderingContext = rc;
-  paintEvent.rect = &r;
+  paintEvent.rect = &fullRect;
+  paintEvent.region = rgn;
 
   mGeckoChild->DispatchWindowEvent(paintEvent);
 
   paintEvent.renderingContext = nsnull;
+  paintEvent.region = nsnull;
 
   targetContext = nsnull;
   targetSurface = nsnull;
@@ -2565,7 +2575,7 @@ NSEvent* globalDragEvent = nil;
 #endif
 
 #else 
-
+  /* non-MOZ_CAIRO_GFX */
   nsCOMPtr<nsIRegion> rgn(do_CreateInstance(kRegionCID));
   if (rgn) {
     rgn->Init();