Pinch is unavailable in a one-handed grip and needs an alternative path

What it is

A pinch needs two fingers on glass at once, opening then closing. In a commute, one hand holds the phone and the other a bag, and the motion cannot be formed. If zoom-out has only pinch, one-handed users are locked at the current scale. The alternative need not copy continuous span. It must be able to make scale smaller, inside the one-thumb reach zone.

Why it happens

A two-finger gesture’s workspace is the convex hull both fingers can hold. In a one-handed grip the thumb’s reach sits on one side of the lower half, the index often on the side or the back, and the hull collapses to one contact. Pinch then fails at the posture layer, not at a recognizer threshold. Double-tap zoom, plus/minus, a slider, or double-tap-then-drag rewrite “change scale” as one contact or a visible control. The alternative trades precision or speed for reach in that grip. With no alternative people pass the phone to the other hand or use a nose, which is not an acceptable strategy.

Studying it

Require “non-dominant hand holds a cup, dominant hand holds the phone,” and complete “fit the whole building in the viewport.” Compare pinch-only, pinch plus double-tap, and pinch plus on-screen buttons. Dependent measures include completion, posture switches, and time. Standing and walking expose failure better than a desk. Do not recruit only young adults who pinch easily.

Where it stops holding

A landscape tablet with both hands already on the glass barely has a one-handed pinch problem. On a watch two fingers cover the dial, so pinch was never the main channel. System assistive touch can map pinch onto a switch, but the app should still ship a visible zoom control and not assume the service is on. Cars and gloves also fail capacitive detection, which is not a finger-count issue.

Applying it

• On maps, photos, and readers, offer one-finger or visible zoom-out: double-tap to fit, plus/minus, or a scale slider, on a thumb-reachable edge.
• Do not keep zoom-out only on a two-finger gesture and hide the button under “more.”
• Finish “see the whole building” with a cup in the other hand. If the cup has to be put down before a pinch is possible, the path is not one-handed.

Related

• Same group: C3.08.1 Scale is set continuously by change in the two-finger span · C3.08.2 The zoom anchor is the two-finger midpoint so content under the pinch does not drift · C3.08.3 Min and max scale should rubber-band at the limit · C3.08.4 Lift keeps the current scale; it does not reset
• Adjacent: C2.11 Thumb reach zones · C3.24 Accessible alternatives to gestures
• Search: one-handed pinch · zoom control · thumb reach