In plain language
Merely focusing (tabbing to) an element must not cause big changes — no popups, no navigation, no focus jumps.
Who it affects
Keyboard and screen-reader users, and people who are easily disoriented by surprise UI changes.
How to check
Tab through everything without pressing Enter: nothing should open, navigate, submit, or move focus by itself. Worked example — A podcast app's episode list: tab to an element and stop: if the page scroll-jacks to a different section automatically, it fails.
Common misconception
Auto-advancing a wizard the moment a radio button is selected feels like a smooth experience for a mouse user, but it can silently submit a choice a keyboard user was still tabbing past. Real example: on an online spreadsheet's toolbar, as soon as an element receives keyboard focus, a script throws focus away (blur) — keyboard users can never actually reach or use it.