Dear ImGui
Bloat-free Graphical User interface for C++ with minimal dependencies
... this goal, and lacks certain features normally found in more high-level libraries. Dear ImGui is particularly suited to integration in games engines (for tooling), real-time 3D applications, fullscreen applications, embedded applications, or any applications on consoles platforms where operating system features are non-standard. The core of Dear ImGui is self-contained within a few platform-agnostic files which you can easily compile in your application/engine.