--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -1891,17 +1891,18 @@ nsresult
 PresShell::ResizeReflowIgnoreOverride(nscoord aWidth, nscoord aHeight, nscoord aOldWidth, nscoord aOldHeight)
 {
   NS_PRECONDITION(!mIsReflowing, "Shouldn't be in reflow here!");
 
   // 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 = mFrameConstructor->GetRootFrame();
-  if (!rootFrame && aHeight == NS_UNCONSTRAINEDSIZE) {
+  if (!rootFrame &&
+      (aHeight == NS_UNCONSTRAINEDSIZE || aWidth == 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;
   }
 
   mPresContext->SetVisibleArea(nsRect(0, 0, aWidth, aHeight));
 
   // There isn't anything useful we can do if the initial reflow hasn't happened.