System gestures truncate same-shape gestures inside the app
Aliases: gesture truncation · pointer cancel · stolen trace
What it is
Even when the app is not binding the official reserved command, a look-alike motion—three-finger pan on a canvas, four-finger pinch to select—can still be judged mid-trace as a shell gesture, and the second half is cut away. The app sees a contact that ends suddenly, not a finished canvas manipulation. Truncation happens on shape similarity, not only when you deliberately steal a reserved command.
Why it happens
The system recognizer watches count, direction, and span rate, not your business intent. A slow three-finger pan on a canvas already matches the prefix of “three-finger swipe to switch apps,” so capture happens before the displacement is done. If the app has already walked a layer tens of pixels, cancel requires a rollback or a misalignment remains. Pinch look-alikes are worse: four fingers closing a little looks like home, and a multi-finger canvas scale is sent to the desktop before the factor is written. The cut point depends on the system’s confidence threshold; the app cannot predict it exactly and must write the state machine as “may break at any time.”
Studying it
On a canvas, do a three-finger pan and a slight four-finger pinch with system gestures left on. Log trace length, displacement at cancel, whether rollback is clean, and whether people thought they finished an app command. Drop to two fingers as a control: same shape, count that does not trigger the shell, and the trace should complete. Comparing cut points across OS versions shows whether the system threshold is tightening.
Where it stops holding
When the system offers an API that yields the content area and keeps only the edges, truncation can step aside by region, but edges are still taken. Rewriting three fingers as two plus a hardware-keyboard modifier avoids the shape clash. A mouse-only session has none of these system touch gestures, so truncation is absent and desktop tests false-negative.
Applying it
- Default canvas manipulation to one or two fingers. If three are truly required, handle mid-gesture cancel: roll back the half-finished work; do not commit a truncated transform.
- Do not write irreversibly while a truncatable gesture is in flight.
- With system gestures on, slowly drag a long three-finger stroke on the canvas. If the person is in multitasking halfway through the line, that is truncation. Check whether the app treated the half line as a finished object or already undid it.