This adds gamepad input via SDL_GameControllerDB, support for Vulkan on macOS via MoltenVK, better handling of high-DPI and scaling, changing attributes of existing windows, raw mouse motion input, explicit support for joystick hats/dpads, user attention requests, transparent windows and framebuffers (where possible), query for monitor work area, more run-time configuration, various other features as well as fixes for a large number of bugs.
See the release notes for more information about the new features and any caveats, deprecations and removals, or the version history for a complete list of changes.