From ec4c95d775be3dea4598bf3756370366c976785e Mon Sep 17 00:00:00 2001
From: "L. David Baron" <dbaron@dbaron.org>
Date: Wed, 8 Oct 2014 14:27:03 -0700
Subject: [PATCH] Bug 1047928 patch 9 - Don't rerun selector matching for
 viewport units.  r=bzbarsky

---
 layout/base/nsPresContext.cpp | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/layout/base/nsPresContext.cpp b/layout/base/nsPresContext.cpp
index 2063441a42c6..60084ab3bb66 100644
--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -1885,19 +1885,27 @@ void
 nsPresContext::MediaFeatureValuesChanged(StyleRebuildType aShouldRebuild,
                                          nsChangeHint aChangeHint)
 {
-  NS_ASSERTION(aShouldRebuild == eAlwaysRebuildStyle || aChangeHint == 0,
-               "If you don't know if we need a rebuild, how can you provide a hint?");
-
   mPendingMediaFeatureValuesChanged = false;
 
-  // MediumFeaturesChanged updates the applied rules, so it always gets called.
-  bool mediaFeaturesDidChange = mShell ? mShell->StyleSet()->MediumFeaturesChanged(this)
-                                       : false;
+  nsRestyleHint aRestyleHint = nsRestyleHint(0);
 
-  if (aShouldRebuild == eAlwaysRebuildStyle ||
-      mediaFeaturesDidChange ||
-      (mUsesViewportUnits && mPendingViewportChange)) {
-    RebuildAllStyleData(aChangeHint, eRestyle_Subtree);
+  if (aShouldRebuild == eAlwaysRebuildStyle) {
+    // FIXME: Pass restyle hint from caller.
+    aRestyleHint |= eRestyle_Subtree;
+  }
+
+  // MediumFeaturesChanged updates the applied rules, so it always gets called.
+  if (mShell && mShell->StyleSet()->MediumFeaturesChanged(this)) {
+    aRestyleHint |= eRestyle_Subtree;
+  }
+
+  if (mUsesViewportUnits && mPendingViewportChange) {
+    // Rebuild all style data without rerunning selector matching.
+    aRestyleHint |= eRestyle_ForceDescendants;
+  }
+
+  if (aRestyleHint || aChangeHint) {
+    RebuildAllStyleData(aChangeHint, aRestyleHint);
   }
 
   mPendingViewportChange = false;