Backdrop now incorporates a touch-friendly and responsive Administrative Bar (menu) in core. Essential features of the Admin Bar:
The toolbar top-level navigation is the root items in the administration menu (like D7 admin_menu).
The toolbar provides hover-over navigation for desktop computers (again like admin_menu), but also behaves like a mobile (hamburger) menu on small screens.
Built-in menu search functionality (separate from main site search, for discovery of menu items rather than content or users).
Extensible to allow other modules to provide other top-level functionality options (like the clear-cache menu or user switcher provided by admin_menu).
The Admin Bar is a port of the Drupal 7 Admin Menu module (https://www.drupal.org/project/admin_menu) with the following changes:
Converted variables to config system.
Merged in the appearance of the "admin_menu_toolbar" submodule into the main module.
Combined the RTL stylesheet with the main stylesheet.
Eliminated use of "hover" in the CSS and JS, since hover events are not supported on touch-devices. Added in event handling for touchstart and touchend, making it so the menu can be operated with touch devices consistently.
Added responsive behavior to window resizing. If the window becomes too narrow, all of the admin menu moves into a single dropdown. Likewise all "extra" functionality (search, user links, logged-in users) collapses when needed.
Tweaked the "search" functionality to improve the UX and work on touch devices.
Removed a bunch of contrib-specific integration, including Devel module integration (user switching), Drupal.org issue queue links, toggling developer modules, and various admin interface "tweaks" that aren't related to the menu at all.
Moved output and theming code from .module to .inc to reduce loading.
Updated tests to accommodate for changes.