Fix the vertical scrollbar guessing in nsHTMLScrollFrame to do better for an

initial reflow if we're doing that initial reflow a ways into the document
load.  Bug 378480, r+sr=roc

content/base/src/nsContentSink.cpp

@@ -892,6 +892,8 @@ nsContentSink::StartLayout(PRBool aIgnorePendingSheets)
     // Nothing to do here
     return;
   }
+
+  PRBool deferredStart = mDeferredLayoutStart;
   
   mDeferredLayoutStart = PR_TRUE;
 
@@ -930,9 +932,12 @@ nsContentSink::StartLayout(PRBool aIgnorePendingSheets)
       // Make shell an observer for next time
       shell->BeginObservingDocument();
 
+      PRBool oldInEagerStartLayout = shell->IsInEagerStartLayout();
+      shell->SetInEagerStartLayout(!deferredStart);
       // Resize-reflow this time
       nsRect r = shell->GetPresContext()->GetVisibleArea();
       nsresult rv = shell->InitialReflow(r.width, r.height);
+      shell->SetInEagerStartLayout(oldInEagerStartLayout);
       if (NS_FAILED(rv)) {
         return;
       }

layout/base/nsIPresShell.h

@@ -720,6 +720,13 @@ public:
   void AddWeakFrame(nsWeakFrame* aWeakFrame);
   void RemoveWeakFrame(nsWeakFrame* aWeakFrame);
 
+  void SetInEagerStartLayout(PRBool aInEagerStartLayout) {
+    mInEagerStartLayout = aInEagerStartLayout;
+  }
+  PRBool IsInEagerStartLayout() const {
+    return mInEagerStartLayout;
+  }
+
 #ifdef NS_DEBUG
   nsIFrame* GetDrawEventTargetFrame() { return mDrawEventTargetFrame; }
 #endif
@@ -760,6 +767,10 @@ protected:
   // the dom/layout trees
   PRPackedBool              mIsAccessibilityActive;
 
+  // True if we're under an "eager" (that is, called before there is
+  // much content in the document) StartLayout call.
+  PRPackedBool              mInEagerStartLayout;
+
   // A list of weak frames. This is a pointer to the last item in the list.
   nsWeakFrame*              mWeakFrames;
 };

layout/generic/nsGfxScrollFrame.cpp

@@ -493,7 +493,7 @@ nsHTMLScrollFrame::GuessVScrollbarNeeded(const ScrollReflowState& aState)
 
   // If this is the initial reflow, guess PR_FALSE because usually
   // we have very little content by then.
-  if (GetStateBits() & NS_FRAME_FIRST_REFLOW)
+  if (InInitialReflow())
     return PR_FALSE;
 
   if (mInner.mIsRoot) {
@@ -517,6 +517,18 @@ nsHTMLScrollFrame::GuessVScrollbarNeeded(const ScrollReflowState& aState)
   return PR_FALSE;
 }
 
+PRBool
+nsHTMLScrollFrame::InInitialReflow() const
+{
+  // We're in an initial reflow if NS_FRAME_FIRST_REFLOW is set, unless we're a
+  // root scrollframe during a non-eager StartLayout call on the presshell.  In
+  // that case we want to skip this clause altogether.
+  return
+    (GetStateBits() & NS_FRAME_FIRST_REFLOW) &&
+    (!mInner.mIsRoot ||
+     PresContext()->PresShell()->IsInEagerStartLayout());
+}
+
 nsresult
 nsHTMLScrollFrame::ReflowContents(ScrollReflowState* aState,
                                   const nsHTMLReflowMetrics& aDesiredSize)
@@ -782,8 +794,7 @@ nsHTMLScrollFrame::Reflow(nsPresContext*           aPresContext,
   aDesiredSize.mOverflowArea = nsRect(0, 0, aDesiredSize.width, aDesiredSize.height);
   FinishAndStoreOverflow(&aDesiredSize);
 
-  if (!(GetStateBits() & NS_FRAME_FIRST_REFLOW) &&
-      !mInner.mHadNonInitialReflow) {
+  if (!InInitialReflow() && !mInner.mHadNonInitialReflow) {
     mInner.mHadNonInitialReflow = PR_TRUE;
     if (mInner.mIsRoot) {
       // For viewports, record whether we needed a vertical scrollbar

layout/generic/nsGfxScrollFrame.h

@@ -397,6 +397,10 @@ protected:
   PRBool IsScrollbarUpdateSuppressed() const {
     return mInner.mSupppressScrollbarUpdate;
   }
+
+  // Return whether we're in an "initial" reflow.  Some reflows with
+  // NS_FRAME_FIRST_REFLOW set are NOT "initial" as far as we're concerned.
+  PRBool InInitialReflow() const;
   
 private:
   friend class nsGfxScrollFrameInner;