On large phones the top bar sits in a hard-to-reach zone
Aliases: one-handed reach · reachability · distal chrome
What it is
As phones grew taller, the top edge left the natural arc of the holding thumb. The thumb zone is the region a person can tap stably with that thumb without changing grip or bringing in the other hand. Back, search, and overflow in the top bar sit outside that arc: visible, hittable in principle, but one-handed only with a stretch or a second hand. The issue is not whether those controls exist. It is that they sit at the far end of the motor range.
Why it happens
In a one-handed grip the palm braces the lower or side edge and the thumb sweeps an arc around the metacarpophalangeal joint. The comfortable band is the lower half and the inner side; the top corners are the far end of the arc and need a straightened thumb or a shift of the phone in the hand. As the diagonal grows, content gets taller and the top bar moves farther from the brace in almost the same proportion. Fitts’s law then sees a larger amplitude against a still-thin target. The same Back control that is easy on a small screen becomes an aiming task that starts with a re-grip.
People compensate: two hands, a fingertip stretch, sliding the phone down before tapping. Compensation has a cost. On a commute, while carrying something, or lying down, it fails, and misses and abandoned actions cluster on the top bar. In apps that already have a bottom tab bar, people prefer the lower edge for “go somewhere else”, and top-bar actions get deferred further.
Studying it
Run a one-handed tapping task: only the dominant thumb, comparing top-bar targets with equivalent targets at the bottom on success, time, and grip adjustments. Independent variables: screen height, corner (top-leading / top-trailing), whether re-gripping is allowed. Dependent variables: first-hit rate, thumb-extension heat maps, and rated effort.
A grip diary fills what the lab misses: have people “go back one level” and “open search” on an actual commute and note whether a second hand appears. Seated lab posture with both hands free underestimates top-bar cost.
Where it stops holding
Two-handed holds, landscape tablets, and a phone lying on a desk being tapped by the other hand shrink the problem. A left-handed grip mirrors the hard zone from top-trailing to top-leading; a right-hand heat map is not a population result. System “reachability” / pull-the-screen-down features temporarily bring the bar into the arc, but they are extra gestures and cannot be assumed on. Pointer-driven desktops have no thumb arc, so the mechanism does not apply. If the bar holds nothing frequent, the cost of distal placement also drops.
Applying it
- Move frequent reach out of the top bar: search, create, and primary destinations into the thumb arc; leave the bar for identification and occasional actions.
- On large phones give Back a path that does not depend on the top-leading corner: an edge swipe, a clear leave control at the bottom.
- Do not put destructive or irreversible actions only in a distal corner; misses while stretching cost more.
- How to check: require one hand for “open a detail and leave” and “open page search”. Watch the recording for re-grips, a second hand, or a fallback to a bottom control. Clustered re-grips mean that action should not live only in the top bar.