Reflow

Adjust the viewport to 1280 pixels wide and then zoom from 100% to 400%. At each zoom level, ensure information is not lost.

No horizontal scrolling is allowed unless required for the content—e.g., a data table or map that is wider than the page (WCAG 2.1 requirement).
Test that dialogs, accordions, tooltips, and menus are usable at up to 400%.