Direction lock removes diagonal drift from natural hand jitter

What it is

Even a subjectively “straight down” scroll carries a few degrees of sideways component. Without a direction lock those components make the list drift while scrolling, so rows fail to stay horizontally registered. The first payoff of the lock is eating physiological jitter; visual neatness is second. It targets unintentional orthogonal noise, not a license to straighten intentional diagonals.

Why it happens

Physiological tremor at rest, roll of the finger pad, and the thumb’s arc all add orthogonal velocity on top of the main-axis motion. Integrated over a screen of travel, that can be a dozen pixels of sideways offset—the column looks wrung. Projection drives the noise’s expectation to zero. This is not a drag-start threshold: that gate decides when a gesture begins; the lock decides where noise goes afterwards. On page indicators and full-width pagers, a few orthogonal pixels already peek the next page, so the lock is close to mandatory.

Studying it

Record the time series of horizontal content offset during “please scroll vertically,” lock on versus off. A known mechanical vibration on the finger can test whether lock knocks down lateral RMS. Measures are orthogonal RMS and “it looks wrung” ratings. Include people with more visible tremor, or healthy-student samples will understate the gain.

Where it stops holding

When jitter is huge, the onset angle itself is polluted and the lock can pick the wrong axis—that is a window that is too short, not an argument against locking. Drawing jitter needs a stabilizer, not a direction lock, or diagonals become axis-aligned. Game crosshairs and map pans need to keep small orthogonal components as fine control. The lock removes unintentional noise; do not file intentional diagonal tasks under “jitter.”

Applying it

• Lock full-width pagers and vertical feeds by default; accept on orthogonal RMS, not frame rate.
• Do not use direction lock as a damper for sparse touch sampling; raise the report rate instead.
• Have someone with an unsteady hand scroll a screen and capture the content path: lateral motion should be nearly a line. If the page wrings, turn the lock on before anything else.

Related

• Same group: C3.28.1 After drag or scroll starts, direction lock snaps later input to the main axis · C3.28.3 Locking too early straightens intended diagonals · C3.28.4 The lock window is the angle of the first pixels of travel
• Adjacent: C2.01 Finger contact area and touch centroid · C3.07 Pan
• Search: hand tremor · orthogonal drift · directional lock