Hidden pref to disable expanding and scrolling to new message when opening folder b=139242 r/sr=bienvenu

mail/base/content/msgMail3PaneWindow.js

@@ -188,7 +188,7 @@ var folderListener = {
              // if we didn't just scroll, 
              // scroll to the first new message
              // but don't select it
-             if (!scrolled)
+             if (!scrolled && pref.getBoolPref("mailnews.scroll_to_new_message"))
                scrolled = ScrollToMessage(nsMsgNavigationType.firstNew, true, false /* selectMessage */);
 
              if (!scrolled && pref.getBoolPref("mailnews.remember_selected_message")) {

mailnews/base/resources/content/msgMail3PaneWindow.js

@@ -194,7 +194,7 @@ var folderListener = {
              // if we didn't just scroll, 
              // scroll to the first new message
              // but don't select it
-             if (!scrolled)
+             if (!scrolled && pref.getBoolPref("mailnews.scroll_to_new_message"))
                scrolled = ScrollToMessage(nsMsgNavigationType.firstNew, true, false /* selectMessage */);
 
              if (!scrolled && pref.getBoolPref("mailnews.remember_selected_message")) {

mailnews/mailnews.js

@@ -291,6 +291,7 @@ pref("mailnews.start_page.url", "chrome://messenger-region/locale/region.propert
 pref("mailnews.start_page.enabled", true);
 
 pref("mailnews.remember_selected_message", true);
+pref("mailnews.scroll_to_new_message", true);
 
 /* file, print, and stop hidden by default.  
    see http://bugzilla.mozilla.org/show_bug.cgi?id=197729#c3 */