diff --git a/layout/base/nsPresShell.cpp b/layout/base/nsPresShell.cpp
index db162814d9a..67c6fe9798a 100644
--- a/layout/base/nsPresShell.cpp
+++ b/layout/base/nsPresShell.cpp
@@ -6582,12 +6582,7 @@ PresShell::DoReflow(nsIFrame* target)
 
   // Don't pass size directly to the reflow state, since a
   // constrained height implies page/column breaking.
-  // Exception: the root frame always uses a constrained reflow
-  nsSize reflowSize;
-  if (target != rootFrame)
-    reflowSize = nsSize(size.width, NS_UNCONSTRAINEDSIZE);
-  else
-    reflowSize = size;
+  nsSize reflowSize(size.width, NS_UNCONSTRAINEDSIZE);
   nsHTMLReflowState reflowState(mPresContext, target, rcx, reflowSize);
 
   // fix the computed height
diff --git a/layout/generic/nsHTMLReflowState.cpp b/layout/generic/nsHTMLReflowState.cpp
index af50f019ab0..28635977f8b 100644
--- a/layout/generic/nsHTMLReflowState.cpp
+++ b/layout/generic/nsHTMLReflowState.cpp
@@ -239,14 +239,10 @@ void
 nsHTMLReflowState::SetComputedHeight(nscoord aComputedHeight)
 {
   NS_ASSERTION(frame, "Must have a frame!");
-  // It'd be nice to assert that |frame| is not in reflow, but this fails for
-  // two reasons:
+  // It'd be nice to assert that |frame| is not in reflow, but this fails
+  // because:
   //
-  // 1) Viewport frames reset the computed height on a copy of their reflow
-  //    state when reflowing fixed-pos kids.  In that case we actually don't
-  //    want to mess with the resize flags, because comparing the frame's rect
-  //    to the munged computed width is pointless.
-  // 2) nsFrame::BoxReflow creates a reflow state for its parent.  This reflow
+  //    nsFrame::BoxReflow creates a reflow state for its parent.  This reflow
   //    state is not used to reflow the parent, but just as a parent for the
   //    frame's own reflow state.  So given a nsBoxFrame inside some non-XUL
   //    (like a text control, for example), we'll end up creating a reflow
@@ -255,9 +251,7 @@ nsHTMLReflowState::SetComputedHeight(nscoord aComputedHeight)
   NS_PRECONDITION(aComputedHeight >= 0, "Invalid computed height");
   if (mComputedHeight != aComputedHeight) {
     mComputedHeight = aComputedHeight;
-    if (frame->GetType() != nsGkAtoms::viewportFrame) { // Or check GetParent()?
-      InitResizeFlags(frame->PresContext());
-    }
+    InitResizeFlags(frame->PresContext());
   }
 }
 
diff --git a/layout/generic/nsHTMLReflowState.h b/layout/generic/nsHTMLReflowState.h
index 19e7ba2f42e..016f81b8017 100644
--- a/layout/generic/nsHTMLReflowState.h
+++ b/layout/generic/nsHTMLReflowState.h
@@ -445,6 +445,14 @@ public:
   // This method doesn't apply min/max computed heights to the value passed in.
   void SetComputedHeight(nscoord aComputedHeight);
 
+  void SetComputedHeightWithoutResettingResizeFlags(nscoord aComputedHeight) {
+    // Viewport frames reset the computed height on a copy of their reflow
+    // state when reflowing fixed-pos kids.  In that case we actually don't
+    // want to mess with the resize flags, because comparing the frame's rect
+    // to the munged computed width is pointless.
+    mComputedHeight = aComputedHeight;
+  }
+
   void SetTruncated(const nsHTMLReflowMetrics& aMetrics, nsReflowStatus* aStatus) const;
 
   PRBool WillReflowAgainForClearance() const {
diff --git a/layout/generic/nsViewportFrame.cpp b/layout/generic/nsViewportFrame.cpp
index a53409f8b66..3b9922f6abc 100644
--- a/layout/generic/nsViewportFrame.cpp
+++ b/layout/generic/nsViewportFrame.cpp
@@ -231,9 +231,8 @@ ViewportFrame::AdjustReflowStateForScrollbars(nsHTMLReflowState* aReflowState) c
     aReflowState->SetComputedWidth(aReflowState->ComputedWidth() -
                                    scrollbars.LeftRight());
     aReflowState->availableWidth -= scrollbars.LeftRight();
-    aReflowState->SetComputedHeight(aReflowState->ComputedHeight() -
-                                    scrollbars.TopBottom());
-    // XXX why don't we also adjust "aReflowState->availableHeight"?
+    aReflowState->SetComputedHeightWithoutResettingResizeFlags(
+      aReflowState->ComputedHeight() - scrollbars.TopBottom());
     return nsPoint(scrollbars.left, scrollbars.top);
   }
   return nsPoint(0, 0);
@@ -278,7 +277,7 @@ ViewportFrame::Reflow(nsPresContext*           aPresContext,
                                          kidFrame, availableSpace);
 
       // Reflow the frame
-      kidReflowState.SetComputedHeight(aReflowState.availableHeight);
+      kidReflowState.SetComputedHeight(aReflowState.ComputedHeight());
       rv = ReflowChild(kidFrame, aPresContext, kidDesiredSize, kidReflowState,
                        0, 0, 0, aStatus);
       kidHeight = kidDesiredSize.height;
@@ -296,8 +295,8 @@ ViewportFrame::Reflow(nsPresContext*           aPresContext,
   aDesiredSize.width = aReflowState.availableWidth;
   // Being flowed initially at an unconstrained height means we should
   // return our child's intrinsic size.
-  aDesiredSize.height = aReflowState.availableHeight != NS_UNCONSTRAINEDSIZE
-                          ? aReflowState.availableHeight
+  aDesiredSize.height = aReflowState.ComputedHeight() != NS_UNCONSTRAINEDSIZE
+                          ? aReflowState.ComputedHeight()
                           : kidHeight;
 
   // Make a copy of the reflow state and change the computed width and height