Menu visibility toggle method
You already mentioned the duplication, so let's address that issue.

First, I'd move the assignment to a common block so it's clear that
you're toggling the visibility.

Then for toggling the CSS class I found the
toggleClass function, so maybe
go with that.

After that moving the assignment to the top, then working based on the
new state gives me this:

onNavTriggerClick: function(event) {
  this._navOpen = !this._navOpen;
  Backbone.Events.trigger(this._navOpen ? 'show:menu' : 'hide:menu');
  $(event.currentTarget).toggleClass('is-active', this._navOpen);
}


Looks a bit more concise.