Sidebar

ui.Sidebar is a source list, as the sidebars of Finder and Mail: items with icons, in sections whose titles hide and show them, choosing the ID of one into a *string. It scrolls when they do not fit. Give it a width:

ui.Sidebar(c, &app.mailbox, func() {
	ui.SidebarSection(c, "Mailboxes", &app.mailboxesShown, func() {
		ui.SidebarItem(c, "inbox", inboxIcon, "Inbox").Children(func() {
			ui.Badge(c, fmt.Sprint(app.unread))
		})
		ui.SidebarItem(c, "sent", sentIcon, "Sent")
	})
	ui.SidebarSection(c, "Tags", nil, func() {
		ui.SidebarItem(c, "work", nil, "Work")
	})
}).Width(220)

• ui.SidebarItem chooses its ID, showing its label after its icon, nil for none; add to it with Children, as a badge.
• ui.SidebarSection holds the items its function builds under a title while its *bool is true: a click on the title shows and hides them, as does its arrow, which shows as the pointer rests on the title. A nil *bool keeps the section open.

Choosing

A click chooses an item; while the sidebar has the focus, Up, Down, Home, End and the first letters of an item choose among them, and Tab leaves it, as it is one stop. Changed reports a new choice. The item chosen shows in the accent color while the sidebar has the focus, in gray otherwise, as AppKit's.

With a router, a *ui.Router in your state, the items' IDs are paths, and a new choice pushes its page:

page := app.router.Path()
if ui.Sidebar(c, &page, func() {
	ui.SidebarItem(c, "/notes", nil, "Notes")
	ui.SidebarItem(c, "/files", nil, "Files")
}).Width(220).Changed() {
	app.router.Push(page)
}

Accessibility

Assistive technology sees a tree: the sections' titles are items that open and close, holding their items, and the item chosen has the focus while the sidebar does.