Taller phones put the top edge out of the thumb's envelope
Aliases: thumb zone · unreachable top · tall-phone reach
What it is
Phones grew taller without growing much wider. In a one-handed grip the thumb's sweep still covers the lower half, so top navigation, close, and back sit outside the arc. That is geometry of a taller phone, not the two-handed scale of a tablet, and not the measurement of the thumb arc as a motor primitive. What used to be reachable near the status bar on a four-inch device now demands a grip shift, the other hand, or giving up on the top chrome. This entry is "the top is gone." It is not the OS mode that slides the UI down, and not where primary actions should be placed.
Why it happens
In a one-handed grip the lower edge sits in the web of the hand and the thumb sweeps from the carpometacarpal joint. Extra height is added almost entirely above that sweep: the home indicator stays inside the arc, each extra strip of top bar is farther away. Width did not scale with height, so the screen did not "get bigger" uniformly; the top was pushed out of the comfortable zone along the long axis. People work around the top-left back control with a bottom edge gesture. Modals, web views, and Camera still put close and done in the top-right corner only. Shifting the grip moves the center of mass and raises drop risk; it is unavailable while walking or while the other hand is holding something. Pixel density does not shorten millimetres. Reach is physical length, not logical resolution.
Studying it
Draw reach maps on real devices in a one-handed grip, not by tapping coordinates on a desktop simulator.
Independent variables: body height, grip (left / right, little-finger shelf or not), target on the top bar versus the bottom bar. Dependent variables: hits without a grip shift, grip-shift count, drops or other-hand rescue, time to finish.
Seated, with an elbow on the table, shrinks the unreachable zone. Include standing and walking. Extreme stretch is not comfort: record "reached but had to move the palm" versus "reached without moving the palm." Do not treat a two-handed tablet heatmap as one-handed phone data.
Where it stops holding
Two-handed use (landscape games, typing) brings the top back; that does not rescue one-handed commuting. On smaller phones, or in larger hands, the top may still sit inside the arc. Hard zones mirror for left and right; moving close from the top-right to the top-left only hands the problem to the other thumb. Cases, rings, and straps change the grip point; redraw the map for the accessory actually used. A system back gesture can replace some top-bar back controls; it cannot replace Done, Close, and Account still pinned at the top.
Applying it
- Treat one-handed commuting as the default posture. On the longest supported device, mark every top-bar control that cannot be hit without a grip shift; do not trust a mock.
- Do not put the only exit or completion in a top corner that requires a grip shift. Provide at least one equivalent path at the bottom or via a gesture.
- Verify on the tallest phone the product claims to support, one-handed, other hand in a pocket: open a modal and dismiss it. If close requires a grip shift or the other hand, the top bar is still being treated as reachable.