Support V4 gets a number of important bug fixes as well as a few new features for NestedScrollView and around MediaSessionCompat.
Media Router has gotten a design overhaul, making connecting and controlling remote devices such as Google Cast enabled devices even easier.
RecyclerView has a significant backward incompatible change to the animation system, offering new flexibility and support for item content animations. You’ll find a SimpleItemAnimator helper class for easing the transition to the new animation system.
AppCompat adds a new tint aware Seekbar, ImageButton, and ImageView and updating styling to SwitchCompat.
Design improves TextInputLayout with support for character counting, the new SCROLL_FLAG_SNAP allows AppBarLayout children to snap completely on or off the screen when scrolling completes, and NavigationView now supports custom views for menu items.
Percent adds support for setting a custom aspect ratio to both PercentFrameLayout and PercentRelativeLayout.
Custom Tabs increases the customization available with the ability to hide the URL bar when scrolling down and change the custom action button for current custom tab session as well as the ability to receive tab shown and hidden events.
Leanback updates GuidedStepFragment so it can be placed on top of existing content and adds new search completion type annotations for SearchFragment.