An on-event handler property that returns false cancels the event, so `onkeydown = (e) => cond && act()` eats every key

Fact

For an event handler set as a property or attribute (el.onkeydown = fn, onclick="..."), a return value of exactly false makes the browser call preventDefault() (for onerror on window, true does). An arrow function written as (e) => e.key === 'Enter' && submit() returns false for every other key. addEventListener callbacks ignore their return value.

Why it matters

The input looks focused and enabled, but nothing typed appears: every keystroke's default action (inserting the character) is cancelled. Synthetic typing in tests fails the same way, so it reads as a focus or automation problem rather than a one-line handler.

How to apply

• Give handler properties a block body, or use addEventListener.
• Lint for it: no-return-assign-style rules do not catch it; a code review rule of "no expression-bodied arrow on an on* property" does.