Immediate mode gui - short imgui. For example the library "Dear imgui".

Basically you write imperative paint functions like this example:

  ImGui::Text("Hello, world %d", 123);
  if (ImGui::Button("Save"))
  {
      // do stuff
  }
  ImGui::InputText("string", buf, IM_ARRAYSIZE(buf));
  ImGui::SliderFloat("float", &f, 0.0f, 1.0f);