--- a/layout/style/ServoBindingList.h
+++ b/layout/style/ServoBindingList.h
@@ -254,17 +254,18 @@ SERVO_BINDING_FUNC(Servo_CSSSupports2, b
                    const nsACString* name, const nsACString* value)
 SERVO_BINDING_FUNC(Servo_CSSSupports, bool,
                    const nsACString* cond)
 
 // Computed style data
 SERVO_BINDING_FUNC(Servo_ComputedValues_GetForAnonymousBox,
                    ServoComputedValuesStrong,
                    ServoComputedValuesBorrowedOrNull parent_style_or_null,
-                   nsIAtom* pseudoTag, RawServoStyleSetBorrowed set)
+                   nsIAtom* pseudoTag, bool skip_display_fixup,
+                   RawServoStyleSetBorrowed set)
 SERVO_BINDING_FUNC(Servo_ComputedValues_Inherit, ServoComputedValuesStrong,
                    RawServoStyleSetBorrowed set,
                    ServoComputedValuesBorrowedOrNull parent_style)
 
 // Initialize Servo components. Should be called exactly once at startup.
 SERVO_BINDING_FUNC(Servo_Initialize, void)
 // Shut down Servo components. Should be called exactly once at shutdown.
 SERVO_BINDING_FUNC(Servo_Shutdown, void)

--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -339,17 +339,17 @@ ServoStyleSet::ResolveInheritingAnonymou
   MOZ_ASSERT(aFlags == 0 ||
              aFlags == nsStyleSet::eSkipParentDisplayBasedStyleFixup);
   bool skipFixup = aFlags & nsStyleSet::eSkipParentDisplayBasedStyleFixup;
 
   const ServoComputedValues* parentStyle =
     aParentContext ? aParentContext->StyleSource().AsServoComputedValues()
                    : nullptr;
   RefPtr<ServoComputedValues> computedValues =
-    Servo_ComputedValues_GetForAnonymousBox(parentStyle, aPseudoTag,
+    Servo_ComputedValues_GetForAnonymousBox(parentStyle, aPseudoTag, skipFixup,
                                             mRawSet.get()).Consume();
 #ifdef DEBUG
   if (!computedValues) {
     nsString pseudo;
     aPseudoTag->ToString(pseudo);
     NS_ERROR(nsPrintfCString("stylo: could not get anon-box: %s",
              NS_ConvertUTF16toUTF8(pseudo).get()).get());
     MOZ_CRASH();
@@ -377,18 +377,22 @@ ServoStyleSet::ResolveNonInheritingAnony
   nsCSSAnonBoxes::NonInheriting type =
     nsCSSAnonBoxes::NonInheritingTypeForPseudoTag(aPseudoTag);
   RefPtr<nsStyleContext>& cache = mNonInheritingStyleContexts[type];
   if (cache) {
     RefPtr<nsStyleContext> retval = cache;
     return retval.forget();
   }
 
+  // We always want to skip parent-based display fixup here.  It never makes
+  // sense for non-inheriting anonymous boxes.
+  MOZ_ASSERT(!nsCSSAnonBoxes::IsNonInheritingAnonBox(nsCSSAnonBoxes::viewport),
+             "viewport needs fixup to handle blockifying it");
   RefPtr<ServoComputedValues> computedValues =
-    Servo_ComputedValues_GetForAnonymousBox(nullptr, aPseudoTag,
+    Servo_ComputedValues_GetForAnonymousBox(nullptr, aPseudoTag, true,
                                             mRawSet.get()).Consume();
 #ifdef DEBUG
   if (!computedValues) {
     nsString pseudo;
     aPseudoTag->ToString(pseudo);
     NS_ERROR(nsPrintfCString("stylo: could not get anon-box: %s",
              NS_ConvertUTF16toUTF8(pseudo).get()).get());
     MOZ_CRASH();