Twenty Nineteen: Mobile menu needs improvement for navigating on touchscreen via keyboard

[laurelfulford]

Copied over from ​https://github.com/WordPress/twentynineteen/issues/723#issuecomment-450173083 (hattip @afercia):

When the "more" menu is expanded on a touchscreen device, there are several hidden tab stops that cause issues when navigating the site via keyboard. These are:

• hidden sub-menu items are still focusable
• all the links in the page are still focusable

When the menu is open it's still possible to tab away from the menu and navigate with the keyboard through all the focusable elements in the page.

If the intent is to show a full-screen menu, the menu should behave like a modal and tabbing should be constrained within the modal.

[afercia]

Although this ticket is up to the Themes team, we've discussed it during today's accessibility bug scrub and agreed to move it to to Future Release as something that needs to be addressed and to clean-up the awaiting review report.