Rotation relocates the one-handed reach zone

What it is

In a one-handed hold the thumb’s arc is pinned to the hand, not to screen coordinates. Rotation moves that arc onto another physical edge: in portrait it usually sits along the bottom toward one side; in landscape it sits along a short edge, toward the bottom of that edge. If “one-handed critical actions” are hard-coded to the lower-right of screen space, after a rotate that corner may be the far top, away from the thumb. The two-handed default still holds. This entry is only how, inside the degraded window, the reach zone follows rotation.

Why it happens

The holding hand grabs a physical edge. Rotation changes which physical edge is “bottom” and which is “side,” and it moves the thumb’s base relative to the pixel grid. Portrait, one hand under the bottom, the arc covers a bottom band. Turn to landscape, the same hand takes a short edge, the arc covers the lower half of that short edge, and the far short edge plus the whole long edge fall outside it. “Bottom” in screen space is then a long edge, no longer the held edge. A toolbar locked to the safe-area bottom may sit in the arc in portrait and, in landscape, stretch along a long edge the thumb cannot reach. Video players step on this: pause on the bottom in portrait, still on “the bottom” after landscape full screen—and that bottom is no longer where the hand is. Rotation can also force a two-handed regrip, so the one-handed window may break for a moment; when one-handed use resumes, critical actions already have to be on the new held edge.

Studying it

Same person, same hand, tap the same one-handed critical actions in portrait and in landscape. Compare “pinned to screen bottom,” “pinned to the holding-hand side,” and “recomputed to the held edge after rotate.”

Independent variables: layout relative to screen coordinates versus to the held edge, whether layout is recomputed after rotate. Dependent variables: whether the first tap after rotate needs a grip change, time, times the device is set down.

Participants should perform the rotation themselves; do not have the facilitator turn the device—the grip change happens during the turn. Do both hands. Do not take gravity’s down as the only definition of “bottom”; in landscape, gravity-bottom and the held edge are often different edges.

Where it stops holding

An orientation-locked app (landscape only, or portrait only) has no such move; side clustering can be laid out for that one orientation. Rotating a device already flat on a desk has no held edge; the problem becomes reading direction, not a thumb arc. A half-folded foldable may not rotate at all, or it becomes two stacked regions; the one-handed arc should be computed on the region still facing the user. If the other hand helps through the rotate and then lets go, there is a two-handed interval in the middle—do not flash the button between sides during the rotation animation.

Applying it

• Walk one-handed critical actions with the held edge, not only with the safe-area “bottom.” After rotate, place them again against the new held edge.
• In a landscape full-screen player, put pause and scrub on the short edge the hand is on, not on the long edge that became the screen bottom.
• Either hand may be the holding hand; after rotate, do not assume lower-right forever.
• Verify by pausing a video one-handed in portrait, rotating to landscape and pausing again, then answering a notification. Any step that needs a hand swap or a set-down means the reach zone is still pinned to an old screen corner.

Related

• Within the group: K3.07.1 Tablet size puts one-handed reach out of bounds; two hands are the default · K3.07.2 Clustering key actions on one edge serves a one-handed thumb · K3.07.4 Designing for degraded one-handed use must not tax two-handed work
• Adjacent: K3.03 Fold-state Transitions · C2.11 Thumb Reach Zones · K1.10 One-handed Mode and Large-screen Reach
• Search terms: orientation reach · rotation grip · thumb zone shift