A new touch should brake immediately
Aliases: scroll intercept · tap-to-stop · cancel inertia
What it is
While content still coasts, a new finger-down must stop the motion on that frame. The down is a brake, not another tap, and not a velocity to add. People use it to catch a list that flew past a target. If the coast continues after down, or the down is delivered as a click on whatever is now underneath, inertia turns from help into harm.
Why it happens
A fling handed control to the simulator. Re-entry means a person wants it back. The right switch is coast → tracking, speed zeroed, content locked to the contact again. If that down is still dispatched to a button, a flying row is clicked under the finger—“I only wanted it to stop” opens an item. If braking eases instead of zeroing, the list slides extra rows and the intercept point misses. A second finger down while another contact is still sliding should also brake, or two speed sources fight.
Studying it
Coast a list at a known v0 and touch down when a named row hits the midline. Measure frames from touch-down to zero speed, the gap between down location and stop location, and whether the row’s click fired. Independent variables include requiring a move before braking, and whether down is handed to hit-testing. Mouse-wheel inertia is a poor baseline: on a desktop, a button-down does not always brake, and copying that logic onto touch is the wrong control.
Where it stops holding
Pull-to-refresh tracking is not intercept: pulling again at the top is overscroll manipulation, not catching a coast. Game camera inertia sometimes ignores taps on purpose so a shot is not interrupted; that is cinematic motion, not an information list. Assistive dwell-click may fire on moving content; suppress clicks during coast until speed is below a threshold and a still interval has passed.
Applying it
- The first touch-down during inertial motion only brakes; it does not fire controls. A real click is a separate tap after content is still.
- Zero speed on the down frame and enter tracking. Do not ease out the remaining travel.
- Fling past a target and tap “stop.” The row that stops should be the row that was under the finger, and it must not open. If content still slides half a screen, or a detail page appears, braking failed.