Immediate mode GUI

• The event processing is directly controlled and completely implemented by the client, and
• The lists of graphical objects to be rendered are kept by the client and all drawing commands required to describe the entire GUI must be re-issued to the graphics pipeline each time a new frame is required.

Widget toolkit

• a GUI needs to be updated in sync with the game scene or complex graphic
• a GUI needs to be overlaid on a game scene or complex graphic
• or a GUI should have an unusual appearance or be pepped with complex graphic

• is more direct in the sense that the widget tree is often a function call tree, that is composable and flexible but hard to interact with
• is less complex and easier to understand - this usually also results in less functionality
• is more elaborate to create and manage if more than a simple widget tree including layout
• has less sophisticated occlusion culling, hit-testing, state change handling, scrolling and focus/hot control animations - this also implies the need to manage logical tree/visual tree itself
• has to rebuild the vertex buffers completely from scratch for each new frame, and thus
• can put a constant work load on the CPU if not utilizing shaders loaded on the GPU

• nuklear with backends for several graphics libraries,
• kiss_sdl for SDL,
• Gio for Go, or
• Dear ImGui written in C++ with back-ends for several graphics APIs, that also has
• * projects for additional back-end support,
• * projects for additional language bindings and
• * ports to other languages.