--- a/layout/base/FrameLayerBuilder.cpp
+++ b/layout/base/FrameLayerBuilder.cpp
@@ -2344,33 +2344,34 @@ ThebesLayerData::Accumulate(ContainerSta
     FLB_LOG_THEBES_DECISION(this, "  Tracking image\n");
   } else if (mImage) {
     FLB_LOG_THEBES_DECISION(this, "  No longer tracking image\n");
     mImage = nullptr;
   }
   bool clipMatches = mItemClip == aClip;
   mItemClip = aClip;
 
-  if (!mIsSolidColorInVisibleRegion && mOpaqueRegion.Contains(aDrawRect) &&
+  nscolor uniformColor;
+  bool isUniform = aItem->IsUniform(aState->mBuilder, &uniformColor);
+
+  if (!mIsSolidColorInVisibleRegion && !isUniform &&
+      mOpaqueRegion.Contains(aDrawRect) &&
       mVisibleRegion.Contains(aVisibleRect)) {
     // A very common case! Most pages have a ThebesLayer with the page
     // background (opaque) visible and most or all of the page content over the
     // top of that background.
     // The rest of this method won't do anything. mVisibleRegion, mOpaqueRegion
     // and mDrawRegion don't need updating. mVisibleRegion contains aVisibleRect
     // already, mOpaqueRegion contains aDrawRect and therefore whatever
     // the opaque region of the item is. mDrawRegion must contain mOpaqueRegion
     // and therefore aDrawRect.
     NS_ASSERTION(mDrawRegion.Contains(aDrawRect), "Draw region not covered");
     return;
   }
 
-  nscolor uniformColor;
-  bool isUniform = aItem->IsUniform(aState->mBuilder, &uniformColor);
-
   // Some display items have to exist (so they can set forceTransparentSurface
   // below) but don't draw anything. They'll return true for isUniform but
   // a color with opacity 0.
   if (!isUniform || NS_GET_A(uniformColor) > 0) {
     // Make sure that the visible area is covered by uniform pixels. In
     // particular this excludes cases where the edges of the item are not
     // pixel-aligned (thus the item will not be truly uniform).
     if (isUniform) {
@@ -2381,16 +2382,21 @@ ThebesLayerData::Accumulate(ContainerSta
         FLB_LOG_THEBES_DECISION(this, "  Display item does not cover the visible rect\n");
       }
     }
     if (isUniform) {
       if (mVisibleRegion.IsEmpty()) {
         // This color is all we have
         mSolidColor = uniformColor;
         mIsSolidColorInVisibleRegion = true;
+      } else if (NS_GET_A(uniformColor) == 255 &&
+                 aVisibleRect.Contains(mVisibleRegion.GetBounds())) {
+        // This color covers everything else in the layer
+        mSolidColor = uniformColor;
+        mIsSolidColorInVisibleRegion = true;
       } else if (mIsSolidColorInVisibleRegion &&
                  mVisibleRegion.IsEqual(nsIntRegion(aVisibleRect)) &&
                  clipMatches) {
         // we can just blend the colors together
         mSolidColor = NS_ComposeColors(mSolidColor, uniformColor);
       } else {
         FLB_LOG_THEBES_DECISION(this, "  Layer not a solid color: Can't blend colors togethers\n");
         mIsSolidColorInVisibleRegion = false;