The decision boundary of a cancellation region must be explicit

What it is

Slide-out cancellation needs an explicit decision boundary: does leaving visual border cancel, leaving an expanded hit area cancel, or must a person enter a dedicated cancel band? An unclear boundary makes lift outcome unpredictable, particularly with small targets, neighbours, and tremor.

Why it happens

The state machine switches candidate and cancellation from the relation between current location and valid region. This actually involves as many as three distinct boundaries, not one: the visually drawn button outline, the hit area expanded outward to meet a minimum touch target size, and a further tolerance buffer added around the hit area to absorb tremor. If all three are implemented as the same line, there is no problem; but if they are independent of each other and not aligned, a transitional zone with poorly defined behaviour opens up between them — a finger resting exactly between the hit-area edge and the tolerance-buffer edge, say, where tiny tremor can flip the state back and forth between candidate and cancelled, and the person sees flickering feedback with no clear sense of which state they are actually in.

Studying it

Test alternative boundary widths, buffers, and neighbour density, recording cancellation success, false commit, re-entry, and judgment of current state. Include fingertip size and tremor rather than validating only ideal simulated traces. One concrete check samples densely specifically within the transitional band between the hit-area edge and the tolerance-buffer edge, and counts how often state flips there — a flip rate noticeably higher than inside or outside that band reveals a gap between the layered boundaries that has not been properly handled.

Where it stops holding

A strict boundary limits false commit but raises false cancel; a huge valid area prevents withdrawal after obvious exit. Continuous drags and sliders need different rules from buttons. The key is not fixed pixels, but stable, intelligible, consequence-appropriate range.

Applying it

• Align cancellation boundary with visible pressed feedback, showing expanded validity when necessary.
• Add small tolerance to prevent tremor flicker while ensuring a real exit cancels high-risk action.
• How to check: sample densely in the transitional band between the hit-area edge and the tolerance-buffer edge and count how often the candidate/cancel state flips there; an excessive flip rate means these layered boundaries need to be merged or realigned, not padded with yet another buffer.

Related

• Same group: C2.08.1 Sliding outside the target then lifting equals cancellation · C2.08.3 Discoverability of cancellation depends on pressed-state feedback
• Nearby: C2.07 Touch-down and touch-up activation · C2.03 Minimum touch-target spacing
• Search terms: cancellation boundary · touch target · forgiveness