Don't let the pres context's visible area get permanently set to an unconstrained height when we call ResizeReflow with an unconstrained height during SizeToContent. b=371508 r+sr=roc a=blocking1.9+

layout/base/nsDocumentViewer.cpp

@@ -3084,11 +3084,10 @@ NS_IMETHODIMP DocumentViewerImpl::SizeToContent()
 
    // so how big is it?
    nsRect shellArea = presContext->GetVisibleArea();
-   if (shellArea.width == NS_UNCONSTRAINEDSIZE ||
-       shellArea.height == NS_UNCONSTRAINEDSIZE) {
    // Protect against bogus returns here
-     return NS_ERROR_FAILURE;
-   }
+   NS_ENSURE_TRUE(shellArea.width != NS_UNCONSTRAINEDSIZE &&
+                  shellArea.height != NS_UNCONSTRAINEDSIZE,
+                  NS_ERROR_FAILURE);
    width = presContext->AppUnitsToDevPixels(shellArea.width);
    height = presContext->AppUnitsToDevPixels(shellArea.height);
 

layout/base/nsPresShell.cpp

@@ -2489,14 +2489,17 @@ NS_IMETHODIMP
 PresShell::ResizeReflow(nscoord aWidth, nscoord aHeight)
 {
   NS_PRECONDITION(!mIsReflowing, "Shouldn't be in reflow here!");
+  NS_PRECONDITION(aWidth != NS_UNCONSTRAINEDSIZE,
+                  "shouldn't use unconstrained widths anymore");
   
   // If we don't have a root frame yet, that means we haven't had our initial
   // reflow... If that's the case, and aWidth or aHeight is unconstrained,
   // ignore them altogether.
   nsIFrame* rootFrame = FrameManager()->GetRootFrame();
 
-  if (!rootFrame &&
-      (aWidth == NS_UNCONSTRAINEDSIZE || aHeight == NS_UNCONSTRAINEDSIZE)) {
+  if (!rootFrame && aHeight == NS_UNCONSTRAINEDSIZE) {
+    // We can't do the work needed for SizeToContent without a root
+    // frame, and we want to return before setting the visible area.
     return NS_ERROR_NOT_AVAILABLE;
   }
 
@@ -2542,6 +2545,11 @@ PresShell::ResizeReflow(nscoord aWidth, nscoord aHeight)
     batch.EndUpdateViewBatch(NS_VMREFRESH_NO_SYNC);
   }
 
+  if (aHeight == NS_UNCONSTRAINEDSIZE) {
+    mPresContext->SetVisibleArea(
+      nsRect(0, 0, aWidth, rootFrame->GetRect().height));
+  }
+
   if (!mIsDestroying) {
     CreateResizeEventTimer();
   }

layout/generic/nsViewportFrame.cpp

@@ -262,7 +262,7 @@ ViewportFrame::Reflow(nsPresContext*          aPresContext,
   // Reflow the main content first so that the placeholders of the
   // fixed-position frames will be in the right places on an initial
   // reflow.
-  nsRect kidRect(0,0,aReflowState.availableWidth,aReflowState.availableHeight);
+  nscoord kidHeight = 0;
 
   nsresult rv = NS_OK;
   
@@ -284,10 +284,11 @@ ViewportFrame::Reflow(nsPresContext*          aPresContext,
       kidReflowState.SetComputedHeight(aReflowState.availableHeight);
       rv = ReflowChild(kidFrame, aPresContext, kidDesiredSize, kidReflowState,
                        0, 0, 0, aStatus);
-      kidRect.width = kidDesiredSize.width;
-      kidRect.height = kidDesiredSize.height;
+      kidHeight = kidDesiredSize.height;
 
       FinishReflowChild(kidFrame, aPresContext, nsnull, kidDesiredSize, 0, 0, 0);
+    } else {
+      kidHeight = mFrames.FirstChild()->GetSize().height;
     }
   }
 
@@ -300,7 +301,7 @@ ViewportFrame::Reflow(nsPresContext*          aPresContext,
   // return our child's intrinsic size.
   aDesiredSize.height = aReflowState.availableHeight != NS_UNCONSTRAINEDSIZE
                           ? aReflowState.availableHeight
-                          : kidRect.height;
+                          : kidHeight;
 
   // Make a copy of the reflow state and change the computed width and height
   // to reflect the available space for the fixed items