To keep the dropdown open while editing, follow next steps:

  1. Open Structure > Go to Mega Menu "X" > Header Row > Row Right > Menu Line > Mega Menu > Drop Down > Select Menu
  2. Click Selectors > Pick oxy-drop-menu class
  3. Go to Advanced tab > Layout > Under Display options uncheck the "none" option

Or, you can watch this short video that will show you how to achieve this: