The idea
Everything at once. A dashboard with a header, a sidebar and a main area that reflows from a single stacked column on phones to a side-by-side layout on desktop, with a second breakpoint that widens the main area further. Match all three regions at every stage width.
Validated at 420px, 560px and 700px — the layout stacks on the narrow stage, goes side-by-side in the middle, and widens the main area on the wide stage.