Form inputs, chrome, and the rest of what you need.
The migration tool's output uses these too. None are flagship features but all are production-ready, typed, and accessible.
Form Inputs
TextInput, Combobox, Checkbox, CheckboxGroup, DatePicker, RichTextEditor, AddNew.
Layout & Chrome
Button, AppHeader, AppFooter, ModalDialog, Toast, ColumnChooser, EditModePanel, HtmlTemplate.
Settings
SettingsPanel handles the getSetting() wiring legacy App SDK widgets relied on, so admins still configure your widget the way they always did.