Bug 523911. Content that's not moving, that's positioned over uniform content that is moving, does not need to be repainted. r=dbaron

layout/base/nsDisplayList.cpp

@@ -324,7 +324,8 @@ nsDisplayList::ComputeVisibility(nsDisplayListBuilder* aBuilder,
   nsAutoTArray<nsDisplayItem*, 512> elements;
   FlattenTo(&elements);
 
-  // Accumulate the bounds of all moving content we find in this list
+  // Accumulate the bounds of all moving content we find in this list.
+  // For speed, we store only a bounding box, not a region.
   nsRect movingContentAccumulatedBounds;
   // Store an overapproximation of the visible regions for the moving
   // content in this list
@@ -345,7 +346,13 @@ nsDisplayList::ComputeVisibility(nsDisplayListBuilder* aBuilder,
 
     nsIFrame* f = item->GetUnderlyingFrame();
     PRBool isMoving = f && aBuilder->IsMovingFrame(f);
-    if (isMoving) {
+    // Record bounds of moving visible items in movingContentAccumulatedBounds.
+    // We do not need to add items that are uniform across the entire visible
+    // area, since they have no visible movement.
+    if (isMoving &&
+        !(item->IsUniform(aBuilder) &&
+          bounds.Contains(aVisibleRegion->GetBounds()) &&
+          bounds.Contains(aVisibleRegionBeforeMove->GetBounds()))) {
       if (movingContentAccumulatedBounds.IsEmpty()) {
         // *aVisibleRegion can only shrink during this loop, so storing
         // the first one we see is a sound overapproximation

layout/base/tests/scrolling_helper.html

@@ -91,6 +91,11 @@ body > div {
   <div style="margin-top:20px; height:20px; margin-bottom:300px; background-color:blue; overflow:hidden;"></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>
+</div>
+
 <script>
 var tests = document.querySelectorAll("body>div");
 var currentTest = -1;
@@ -227,6 +232,13 @@ function testMovingClipArea(blitRegion, paintRegion) {
      "Shouldn't repaint anything: " + 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());
+  ok(paintRegion.isEmpty(),
+     "Shouldn't repaint anything: " + paintRegion.toString());
+}
+
 function clientRectToRect(cr)
 {
   return [cr.left, cr.top, cr.right, cr.bottom];