Pico GUI library is based around the PicoGraphics library. It is currently tested/developed for the Pico Display Pack, however it should work with any display (possibly with a few minor alterations).
The gui is made up of a few main components. The ViewController which handles events and manages which View is currently being displayed. Views are just specialised Widgets with a few extra features. The most vital component is probably the Widget class. Every GUI object is an instance of a Widget (And Drawable too).
The Widget class is the backbone of this library. It manages parents, children, and a bunch more too. Every Widget has an associated layout which is made up of a Box (It's actual position and size on the display), a Size (the minimum space a widget requires) and a boolean denoting whether this Widget is growable (woild like to take up an arbitrary amount of space).
Each Widget is responsible for managing its own layout and children if it is moved or re-sized. This is done using the bool Widget::relayout() method which is called by the ViewController (or parent widget in its relayout()) when the layout has been invalidated by invoking Drawable::requiresRelayout().
Each Widget is responsible for redrawing itself and instructing its children to redraw. This is done using the void Widget::redraw() method which is called by the ViewController (or parent widget in its relayout()) when the graphics has been invalidated by invoking Drawable::requiresRedraw().
When implementing a new Widget, the two most important parts are ensuring it is drawn correctly (not exceeding its Box etc) and ensuring that the layout of any children it may have is correct (spaced and offset correctly from each other)
The View class is what a developer wishing to create a new screen/scene/view/whatever-you-want-to-call-it will inherit from. This has a few added features for managing Input and other Events.
The ViewController class is responsible for managing view changes, events (e.g. user input, timer), instructing the current view to redraw() or relayout(). You should not need to modify this unless you are adding functionality.