--- a/layout/base/FrameLayerBuilder.cpp
+++ b/layout/base/FrameLayerBuilder.cpp
@@ -1496,21 +1496,52 @@ ContainerState::FindOpaqueBackgroundColo
     intersection.And(candidate->mVisibleRegion, target->mVisibleRegion);
     if (intersection.IsEmpty()) {
       // The layer doesn't intersect our target, ignore it and move on
       continue;
     }
 
     // The candidate intersects our target. If any layer has a solid-color
     // area behind our target, this must be it. Scan its display items.
-    nsRect rect =
-      target->mVisibleRegion.GetBounds().ToAppUnits(mAppUnitsPerDevPixel);
-    rect.ScaleInverseRoundOut(mParameters.mXScale, mParameters.mYScale);
-    return mLayerBuilder->
-      FindOpaqueColorCovering(mBuilder, candidate->mLayer, rect);
+    nsIntRect deviceRect = target->mVisibleRegion.GetBounds();
+    nsRect appUnitRect = deviceRect.ToAppUnits(mAppUnitsPerDevPixel);
+    appUnitRect.ScaleInverseRoundOut(mParameters.mXScale, mParameters.mYScale);
+
+    FrameLayerBuilder::ThebesLayerItemsEntry* entry =
+      mLayerBuilder->GetThebesLayerItemsEntry(candidate->mLayer);
+    NS_ASSERTION(entry, "Must know about this layer!");
+    for (int32_t j = entry->mItems.Length() - 1; j >= 0; --j) {
+      nsDisplayItem* item = entry->mItems[j].mItem;
+      bool snap;
+      nsRect bounds = item->GetBounds(mBuilder, &snap);
+      if (snap && mSnappingEnabled) {
+        nsIntRect snappedBounds = ScaleToNearestPixels(bounds);
+        if (!snappedBounds.Intersects(deviceRect))
+          continue;
+
+        if (!snappedBounds.Contains(deviceRect))
+          break;
+
+      } else {
+        // The layer's visible rect is already (close enough to) pixel
+        // aligned, so no need to round out and in here.
+        if (!bounds.Intersects(appUnitRect))
+          continue;
+
+        if (!bounds.Contains(appUnitRect))
+          break;
+      }
+
+      nscolor color;
+      if (item->IsUniform(mBuilder, &color) && NS_GET_A(color) == 255)
+        return color;
+
+      break;
+    }
+    break;
   }
   return NS_RGBA(0,0,0,0);
 }
 
 void
 ContainerState::ThebesLayerData::UpdateCommonClipCount(
     const FrameLayerBuilder::Clip& aCurrentClip)
 {
@@ -2608,38 +2639,16 @@ FrameLayerBuilder::SaveLastPaintOffset(T
     if (entry->mContainerLayerGeneration == 0) {
       entry->mContainerLayerGeneration = mContainerLayerGeneration;
     }
     entry->mLastPaintOffset = GetTranslationForThebesLayer(aLayer);
     entry->mHasExplicitLastPaintOffset = true;
   }
 }
 
-nscolor
-FrameLayerBuilder::FindOpaqueColorCovering(nsDisplayListBuilder* aBuilder,
-                                           ThebesLayer* aLayer,
-                                           const nsRect& aRect)
-{
-  ThebesLayerItemsEntry* entry = mThebesLayerItems.GetEntry(aLayer);
-  NS_ASSERTION(entry, "Must know about this layer!");
-  for (int32_t i = entry->mItems.Length() - 1; i >= 0; --i) {
-    nsDisplayItem* item = entry->mItems[i].mItem;
-    const nsRect& visible = item->GetVisibleRect();
-    if (!visible.Intersects(aRect))
-      continue;
-
-    nscolor color;
-    if (visible.Contains(aRect) && item->IsUniform(aBuilder, &color) &&
-        NS_GET_A(color) == 255)
-      return color;
-    break;
-  }
-  return NS_RGBA(0,0,0,0);
-}
-
 void
 ContainerState::CollectOldLayers()
 {
   for (Layer* layer = mContainerLayer->GetFirstChild(); layer;
        layer = layer->GetNextSibling()) {
     NS_ASSERTION(!layer->HasUserData(&gMaskLayerUserData),
                  "Mask layer in layer tree; could not be recycled.");
     if (layer->HasUserData(&gThebesDisplayItemLayerUserData)) {

--- a/layout/base/FrameLayerBuilder.h
+++ b/layout/base/FrameLayerBuilder.h
@@ -301,24 +301,16 @@ public:
                         nsDisplayItemGeometry** aOldGeometry = nullptr, 
                         Clip** aOldClip = nullptr,
                         nsTArray<nsIFrame*>* aChangedFrames = nullptr,
                         bool *aIsInvalid = nullptr);
 
   static Layer* GetDebugOldLayerFor(nsIFrame* aFrame, uint32_t aDisplayItemKey);
 
   /**
-   * Try to determine whether the ThebesLayer aLayer paints an opaque
-   * single color everywhere it's visible in aRect.
-   * If successful, return that color, otherwise return NS_RGBA(0,0,0,0).
-   */
-  nscolor FindOpaqueColorCovering(nsDisplayListBuilder* aBuilder,
-                                  ThebesLayer* aLayer, const nsRect& aRect);
-
-  /**
    * Destroy any stored LayerManagerDataProperty and the associated data for
    * aFrame.
    */
   static void DestroyDisplayItemDataFor(nsIFrame* aFrame);
 
   LayerManager* GetRetainingLayerManager() { return mRetainingManager; }
 
   /**