Header is an app-chrome control (top area) that can display left/right aligned content like title, breadcrumbs, and key hints.
Header is typically used as the top bar of a DockLayout.
var root = new DockLayout()
.Top(new Header()
.Left("My App")
.Right(new CommandBar()))
.Content(mainContent);
HorizontalAlignment = Align.Stretch, VerticalAlignment = Align.StartLeft, Center, Right: slot visuals arranged on a single row