TachyonUI is an immediate-mode C++20 GUI library for tools:

You build the UI in OnGUI(), a function of your app that runs every frame.
The library keeps the state between frames, lays the UI out and draws it on the GPU.

void OnGUI() override
{
    if (BeginWindow(windowID, "Hello", LayoutOverride{}.Size(360, 160)))
        Button("Quit", [this] { applicationQuit = true; });
    EndWindow();
}

New? Start with the Tutorial.

Guide	Covers
Tutorial	Your first app, step by step
Overview	The frame, IDs, Begin/End, return values, units, errors, saved state
Controls	Every control: signatures, return values, CSS classes
CSS and theming	Themes, selectors, values, properties
Integration	Build setup, your own Vulkan device, window, input and loop
Examples	The example programs

API reference: every class and function, also grouped by topic.

Tutorial: your first app Overview Controls CSS and theming Integration Examples