Opening the menu by clicking a button.

The menu can be opened by a button(or other element) by configuring the following options:

• openBelowContext: true, this causes the contextmenu to open at the lowerleft corner of the context.
• event: click, this is the event used to open the menu, i.e. clicking the button.

The options.startLeftOffset and option.startTopOffset properties can be used to offset the position of the menu
relative to the lowerleft corner of it's context.

Dropdown menu >> << onShow callback alternative