Indicates how the menu width and the menupopup width are determined. If the sizetopopup attribute is left out or set to none, the menu will be its preferred width and the popup may extend outside of this width, unaffected by the maximum width of the menu itself. Otherwise, the menu will size itself to at least the size of the popup. If the menu has a maximum width, the popup will also be this width.
The width of the popup will not be constrained to the size of the menu.
The preferred width of the menu label or button will be the size needed for the popup contents. This is the default value for menulists.
Both the preferred and minimum width of the menu label or button will be the same as that necessary for the menupopup.