fixed bug 1147307 (Transtion + fast slide changes + ESC causes screen to open again)

Fixes: https://launchpad.net/bugs/1147307

openlp/core/ui/maindisplay.py

@@ -362,6 +362,7 @@ class MainDisplay(Display):
         Generates a preview of the image displayed.
         """
         log.debug(u'preview for %s', self.isLive)
+        was_visible = self.isVisible()
         Receiver.send_message(u'openlp_process_events')
         # We must have a service item to preview.
         if self.isLive and hasattr(self, u'serviceItem'):
@@ -380,7 +381,8 @@ class MainDisplay(Display):
         if self.isLive:
             if self.hideMode:
                 self.hideDisplay(self.hideMode)
-            else:
+            # Only continue if the visibility wasn't changed during method call.
+            elif was_visible == self.isVisible():
                 # Single screen active
                 if self.screens.display_count == 1:
                     # Only make visible if setting enabled.