Back out the rest of bug 72747 due to Tp regression.

content/xml/document/src/nsXMLContentSink.cpp

@@ -78,6 +78,7 @@
 #include "prlog.h"
 #include "prmem.h"
 #include "nsParserUtils.h"
+#include "nsIScrollable.h"
 #include "nsRect.h"
 #include "nsGenericElement.h"
 #include "nsIWebNavigation.h"
@@ -831,6 +832,12 @@ nsXMLContentSink::PopContent()
 void
 nsXMLContentSink::StartLayout()
 {
+  // Reset scrolling to default settings for this shell.
+  // This must happen before the initial reflow, when we create the root frame
+  nsCOMPtr<nsIScrollable> scrollableContainer(do_QueryInterface(mDocShell));
+  if (scrollableContainer) {
+    scrollableContainer->ResetScrollbarPreferences();
+  }
   PRBool topLevelFrameset = PR_FALSE;
   nsCOMPtr<nsIDocShellTreeItem> docShellAsItem(do_QueryInterface(mDocShell));
   if (docShellAsItem) {