diff --git a/mozilla/view/public/nsIView.h b/mozilla/view/public/nsIView.h
index 86545a25b74..99aac79ec92 100644
--- a/mozilla/view/public/nsIView.h
+++ b/mozilla/view/public/nsIView.h
@@ -278,9 +278,13 @@ public:
   PRBool IsTransparent() const { return (mVFlags & NS_VIEW_FLAG_TRANSPARENT) != 0; }
 
   /**
-   * Indicate that this view is always painted onto a uniform field of pixels. Thus,
-   * even if the view is transparent, it may still be bitblit scrollable because
-   * the background that shines through does not vary with position.
+   * Indicate that this view is always painted onto a uniform field of
+   * pixels. Thus, even if the view is transparent, it may still be
+   * bitblit scrollable because the background that shines through
+   * does not vary with position.  Caller must ensure that the pixel
+   * field belongs to the same element as this view or some ancestor
+   * element, so that if the pixel field is in some opacity group, then
+   * this view is also in the opacity group (or some subgroup).
    */
   void SetHasUniformBackground(PRBool aUniform) {
     if (aUniform) {
diff --git a/mozilla/view/src/nsViewManager.cpp b/mozilla/view/src/nsViewManager.cpp
index 1c93c6db725..9415fa93f2f 100644
--- a/mozilla/view/src/nsViewManager.cpp
+++ b/mozilla/view/src/nsViewManager.cpp
@@ -3653,7 +3653,17 @@ void nsViewManager::OptimizeDisplayList(const nsVoidArray* aDisplayList, const n
 
         // a view is opaque if it is neither transparent nor transluscent
         if (!(element->mFlags & (VIEW_TRANSPARENT | VIEW_TRANSLUCENT))
-            || (element->mView->HasUniformBackground() && aTreatUniformAsOpaque)) {
+            // also, treat it as opaque if it's drawn onto a uniform background
+            // and we're doing scrolling analysis; if the background is uniform,
+            // we don't care what's under it. But the background might be translucent
+            // because of some enclosing opacity group, in which case we do care
+            // what's under it. Unfortunately we don't know exactly where the
+            // background comes from ... it may not even have a view ... but
+            // the side condition on SetHasUniformBackground ensures that
+            // if the background is translucent, then this view is also marked
+            // translucent.
+            || (element->mView->HasUniformBackground() && aTreatUniformAsOpaque
+                && !(element->mFlags & VIEW_TRANSLUCENT))) {
           aOpaqueRegion.Or(aOpaqueRegion, element->mBounds);
         }
       }