In addition to lots of bug fixes, here are the major changes in this release:

General:

• The SDL wiki documentation and development headers are automatically kept in sync
• Each function has information about in which version of SDL it was introduced
• SDL-specific CMake options are now prefixed with 'SDL_'. Be sure to update your CMake build scripts accordingly!
• Added the hint SDL_HINT_APP_NAME to let SDL know the name of your application for various places it might show up in system information
• Added SDL_RenderGeometry() and SDL_RenderGeometryRaw() to allow rendering of arbitrary shapes using the SDL 2D render API
• Added SDL_SetTextureUserData() and SDL_GetTextureUserData() to associate application data with an SDL texture
• Added SDL_RenderWindowToLogical() and SDL_RenderLogicalToWindow() to convert between window coordinates and logical render coordinates
• Added SDL_RenderSetVSync() to change whether a renderer present is synchronized with vblank at runtime
• Added SDL_PremultiplyAlpha() to premultiply alpha on a block of SDL_PIXELFORMAT_ARGB8888 pixels
• Added a window event SDL_WINDOWEVENT_DISPLAY_CHANGED which is sent when a window changes what display it's centered on
• Added SDL_GetWindowICCProfile() to query a window's ICC profile, and a window event SDL_WINDOWEVENT_ICCPROF_CHANGED that is sent when it changes
• Added the hint SDL_HINT_VIDEO_EGL_ALLOW_TRANSPARENCY to allow EGL windows to be transparent instead of opaque
• SDL_WaitEvent() has been redesigned to use less CPU in most cases
• Added SDL_SetWindowMouseRect() and SDL_GetWindowMouseRect() to confine the mouse cursor to an area of a window
• You can now read precise mouse wheel motion using 'preciseX' and 'preciseY' event fields
• Added SDL_GameControllerHasRumble() and SDL_GameControllerHasRumbleTriggers() to query whether a game controller supports rumble
• Added SDL_JoystickHasRumble() and SDL_JoystickHasRumbleTriggers() to query whether a joystick supports rumble
• SDL's hidapi implementation is now available as a public API in SDL_hidapi.h

Windows:

• Improved relative mouse motion over Windows Remote Desktop
• Added the hint SDL_HINT_IME_SHOW_UI to show native UI components instead of hiding them (defaults off)

Windows/UWP:

• WGI is used instead of XInput for better controller support in UWP apps

Linux:

• Added the hint SDL_HINT_SCREENSAVER_INHIBIT_ACTIVITY_NAME to set the activity that's displayed by the system when the screensaver is disabled
• Added the hint SDL_HINT_LINUX_JOYSTICK_CLASSIC to control whether /dev/input/js* or /dev/input/event* are used as joystick devices
• Added the hint SDL_HINT_JOYSTICK_DEVICE to allow the user to specify devices that will be opened in addition to the normal joystick detection
• Added SDL_LinuxSetThreadPriorityAndPolicy() for more control over a thread priority on Linux

Android:

• Added support for audio output and capture using AAudio on Android 8.1 and newer
• Steam Controller support is disabled by default, and can be enabled by setting the hint SDL_HINT_JOYSTICK_HIDAPI_STEAM to "1" before calling SDL_Init()

Apple Arcade:

• Added SDL_GameControllerGetAppleSFSymbolsNameForButton() and SDL_GameControllerGetAppleSFSymbolsNameForAxis() to support Apple Arcade titles

iOS:

• Added documentation that the UIApplicationSupportsIndirectInputEvents key must be set to true in your application's Info.plist in order to get real Bluetooth mouse events.
• Steam Controller support is disabled by default, and can be enabled by setting the hint SDL_HINT_JOYSTICK_HIDAPI_STEAM to "1" before calling SDL_Init()