--- a/xpfe/appshell/nsXULWindow.cpp
+++ b/xpfe/appshell/nsXULWindow.cpp
@@ -2256,18 +2256,22 @@ nsXULWindow::ApplyChromeFlags()
   IgnoredErrorResult rv;
   window->SetAttribute(NS_LITERAL_STRING("chromehidden"), newvalue, rv);
 }
 
 NS_IMETHODIMP
 nsXULWindow::BeforeStartLayout()
 {
   ApplyChromeFlags();
-  SyncAttributesToWidget();
-  SizeShell();
+  // FIXME(emilio): Until we fix bug 1446264, just behind a hidden pref for
+  // local testing and debugging.
+  if (Preferences::GetBool("xul.size_window_before_layout")) {
+    SyncAttributesToWidget();
+    SizeShell();
+  }
   return NS_OK;
 }
 
 void
 nsXULWindow::SizeShell()
 {
   int32_t specWidth = -1, specHeight = -1;
   bool gotSize = false;