From: L. David Baron <dbaron@dbaron.org>

Bug 828312, patch 3: Use the non-inherited hints from the parent style context when that parent style context is on a child frame.

diff --git a/layout/base/RestyleManager.cpp b/layout/base/RestyleManager.cpp
--- a/layout/base/RestyleManager.cpp
+++ b/layout/base/RestyleManager.cpp
@@ -2144,20 +2144,28 @@ ElementRestyler::RestyleSelf(nsRestyleHi
   const nsCSSPseudoElements::Type pseudoType = oldContext->GetPseudoType();
 
   nsStyleContext* parentContext;
   // Get the frame providing the parent style context.  If it is a
   // child, then resolve the provider first.
   nsIFrame* providerFrame = mFrame->GetParentStyleContextFrame();
   bool isChild = providerFrame && providerFrame->GetParent() == mFrame;
   if (!isChild) {
-    if (providerFrame)
+    if (providerFrame) {
       parentContext = providerFrame->StyleContext();
-    else
+
+      if (providerFrame != mFrame->GetParent()) {
+        // We don't actually know what the parent style context's
+        // non-inherited hints were, so assume the worst.
+        mParentFrameHintsNotHandledForDescendants =
+          nsChangeHint_Hints_NotHandledForDescendants;
+      }
+    } else {
       parentContext = nullptr;
+    }
   }
   else {
     MOZ_ASSERT(providerFrame->GetContent() == mFrame->GetContent(),
                "Postcondition for GetParentStyleContextFrame() violated. "
                "That means we need to add the current element to the "
                "ancestor filter.");
 
     // resolve the provider here (before mFrame below).
@@ -2168,32 +2176,27 @@ ElementRestyler::RestyleSelf(nsRestyleHi
     // can't be trusted because it assumes any changes to the parent
     // style context provider will be automatically propagated to
     // the frame(s) with child style contexts.
 
     ElementRestyler providerRestyler(PARENT_CONTEXT_FROM_CHILD_FRAME,
                                      *this, providerFrame);
     providerRestyler.Restyle(aRestyleHint);
     assumeDifferenceHint = providerRestyler.HintsHandledForFrame();
+    mParentFrameHintsNotHandledForDescendants =
+      providerRestyler.HintsNotHandledForDescendants();
 
     // The provider's new context becomes the parent context of
     // mFrame's context.
     parentContext = providerFrame->StyleContext();
     // Set |mResolvedChild| so we don't bother resolving the
     // provider again.
     mResolvedChild = providerFrame;
   }
 
-  if (providerFrame != mFrame->GetParent()) {
-    // We don't actually know what the parent style context's
-    // non-inherited hints were, so assume the worst.
-    mParentFrameHintsNotHandledForDescendants =
-      nsChangeHint_Hints_NotHandledForDescendants;
-  }
-
 #ifdef DEBUG
   {
     // Check that our assumption that continuations of the same
     // pseudo-type and with the same style context parent have the
     // same style context is valid before the reresolution.  (We need
     // to check the pseudo-type and style context parent because of
     // :first-letter and :first-line, where we create styled and
     // unstyled letter/line frames distinguished by pseudo-type, and