This should be an extension, using a element as the menu.

Hint: this menu is in z-index -1.

Opening the menu should be only adding a classname to the body (using emy.addClass(document.body, 'menuisopen') )
Then based on the [selected="true"] attribute, the current view might get a CSS transform translateX(80%).

Closing the shoud be just removing the classname to the body element (using emu.changeClass(document.body, 'menuisopen','')

Menu items should be normal elements inside a list, with onclick href & target taken in account just like any other link.
When a click is triggered, if href is defined, menu has to close first, then trigger the href value.

Best practice is a JS object as sideMenu inside emy.plugin (aka emy.plugin.sideMenu)