Bug 528604. Don't add areas-to-repaint when scrolling for non-moving clip rects that aren't clipping any moving content. r=dbaron

layout/base/nsLayoutUtils.cpp

@@ -1180,8 +1180,12 @@ AddItemsToRegion(nsDisplayListBuilder* aBuilder, nsDisplayList* aList,
         // If the clipping frame is moving, then it isn't clipping any
         // non-moving content (see ApplyAbsPosClipping), so we don't need
         // to do anything special, but we should not restrict aClipRect.
+        // If the clipping frame is not moving, but the moving frames
+        // are not in its descendants, then again we don't need to
+        // do anything special.
         nsIFrame* clipFrame = clipItem->GetClippingFrame();
-        if (!aBuilder->IsMovingFrame(clipFrame)) {
+        if (!aBuilder->IsMovingFrame(clipFrame) &&
+            nsLayoutUtils::IsProperAncestorFrame(clipFrame, aBuilder->GetRootMovingFrame())) {
           nscoord appUnitsPerDevPixel = clipFrame->PresContext()->AppUnitsPerDevPixel();
           // We know the nsDisplayClip will snap because we're in a context
           // where pixels can be blitted and we don't traverse down through

layout/base/tests/scrolling_helper.html

@@ -91,6 +91,13 @@ body > div {
   <div style="margin-top:20px; height:20px; margin-bottom:300px; background-color:blue; overflow:hidden;"></div>
 </div>
 
+<div id="testNonmovingClipArea">
+  <div style="height:300px; background:-moz-linear-gradient(top, red, black);"></div>
+  <div style="position:fixed; left:0; top:150px; width:200px; height:50px; background:yellow; overflow:hidden; opacity:0.5;">
+    <div style="height:50px; background:-moz-linear-gradient(top, green, yellow); opacity:0.3;"></div>
+  </div>
+</div>
+
 <div id="testFixedOverUniform">
   <div style="height:300px; background:blue;"></div>
   <div style="position:fixed; left:0; top:50px; width:100px; height:45px; background:yellow; opacity:0.5;"></div>
@@ -232,6 +239,13 @@ function testMovingClipArea(blitRegion, paintRegion) {
      "Shouldn't repaint anything: " + paintRegion.toString());
 }
 
+function testNonmovingClipArea(blitRegion, paintRegion) {
+  ok(blitRegion.equalsRegion(new Region([[0,0,200,130]])),
+     "Should only blit necessary strip: " + blitRegion.toString());
+  ok(paintRegion.equalsRegion(new Region([[0,130,200,200]])),
+     "Should only repaint necessary strip: " + paintRegion.toString());
+}
+
 function testFixedOverUniform(blitRegion, paintRegion) {
   ok(!blitRegion.intersectsRect([0,20,200,200]),
      "Shouldn't blit anything except possibly to cover the area that has moved offscreen: " + blitRegion.toString());