ppy/osu-framework is a high-performance, cross-platform game framework written in C#, primarily designed to power the rhythm game osu! but offering general-purpose 2D game development capabilities. The codebase provides a comprehensive suite of subsystems including a hardware-accelerated rendering pipeline (OpenGL/Veldrid), a sophisticated input handling system (mouse, keyboard, touch, tablet, joystick, MIDI), an audio engine built on BASS and FFmpeg, a rich animation and layout system, and a full-featured UI toolkit with containers, text rendering, and interactive controls. It employs Roslyn source generators to eliminate runtime reflection for dependency injection and input dispatch. The architecture is modular, heavily test-driven (thousands of unit and visual tests), and supports Windows, macOS, Linux, iOS, and Android through platform-specific host implementations. Despite its strong engineering, the codebase shows signs of ongoing migration to SDL3, dual rendering backends, and reliance on unmanaged native libraries that require periodic updates.