The gesture is a second down that does not lift, then a vertical drag

What it is

Double-tap-drag zoom is not two taps, and not an ordinary drag. It is built as: a short first contact that lifts, then a second down that does not lift, converted into vertical travel. If that second down lifts at once, it is a normal double-tap (one discrete zoom step). If it moves without lifting, the recognizer switches to continuous zoom. The compound lives in the life of the second contact; the first only arms the recognizer.

Why it happens

The first tap puts the state machine in “possible double-tap.” At the second down the system cannot yet tell “tap again” from “hold and drag.” Lift with tiny travel → double-tap completes; travel past a drag gate without lift → drag-zoom, double-tap fails. The gesture therefore parasitizes the double-tap recognizer: without the first tap, a press-and-drag is only a pan. State must span two contacts; any button that handles clicks independently cannot host the chain.

Studying it

Score three segments separately: whether the first tap was remembered, whether the second down attached, whether vertical travel was read as zoom rather than pan. Factors include the gap between taps and how soon movement starts after the second down. Measures include classification as two singles, as pan, or as discrete double-tap. Measuring only “did it zoom” hides which joint broke.

Where it stops holding

A view that does not recognize double-tap cannot arm the chain. If the first tap already navigated (opened a detail), the second down lands on another surface and the compound snaps. Stylus double-tap gaps are tighter and break the chain more easily. This leaf defines composition only, not whether up should zoom in, and not why maps love it.

Applying it

• Make the double-tap recognizer continuable: a second down may enter drag instead of committing a discrete zoom.
• Do not jump scale after the second down until travel crosses the gate, or the drag is preempted as a double-tap.
• Script “tap, tap, drag” versus “tap, tap, lift” and assert that only the former enters continuous zoom.

Related

• Same group: C3.26.2 Vertical motion maps to zoom in or out, sparing a second finger · C3.26.3 The gesture is almost undiscoverable without teaching · C3.26.4 Maps use it as a one-handed stand-in for pinch
• Adjacent: C3.02 Double tap · C3.08 Pinch-to-zoom out
• Search: double-tap drag · tap-hold-drag zoom · compound gesture