[atlas] fix main canvas refresh with dropdown list (fix #14358)

Author: nirvn

src/app/composer/qgscomposer.cpp

@@ -1192,7 +1192,10 @@ void QgsComposer::atlasPageComboEditingFinished()
   }
   else if ( page != mComposition->atlasComposition().currentFeatureNumber() + 1 )
   {
+    mapCanvas()->stopRendering();
+    loadAtlasPredefinedScalesFromProject();
     mComposition->atlasComposition().prepareForFeature( page - 1 );
+    emit atlasPreviewFeatureChanged();
   }
 }