Ensure keyboard focus stays within menus/modal dialogs unless explicitly dismissed
Submitted by Greg Gay: Once a menu or modal dialog is opened it is possible to navigate and submit the form, though when navigating through it focus leaves the dialog and returns to the content in behind it. Focus should remain in popup dialogs until manually dismissed (i.e. Esc), or the login or register link etc. are pressed.
Ref: https://www.w3.org/TR/wai-aria-practices-1.1/examples/dialog-modal/dialog.html