Every function must remain reachable when the pen is absent
Aliases: pen optional · missing stylus · stylus-free path
What it is
A pen is a detachable accessory: battery dead, unpaired, lent, left in another bag. With no pen, the product must still finish every task—an equivalent path through fingers or other input, not a lock of critical capability onto pen events. The division of labour describes how work goes faster with a pen; it must not describe which functions vanish without one. Smooth degradation after a pairing drop is a channel-layer issue; this claim is whether the function still exists.
Why it happens
Commands bound only to pen-specific events (a menu that appears on hover, an eraser only on a barrel button, a hairline that only pressure can draw) lose their entry when the pen is gone. A finger cannot hit a hover hot zone or fake a barrel button. Reachability means every task has a path that does not need pen hardware: an eraser on the toolbar, a width slider instead of pressure, a long-press instead of a hover menu. The path may be slower and coarser; it may not be missing. Shared devices make this stricter: a classroom tablet often has one pen among many, and whoever lacks it can only watch. Accessibility is the same: some people cannot use nib precision but still need the same markup. Detecting “pen not present” should switch layout (commands hidden in hover come out on the toolbar), not leave an empty pen cursor that implies writing still works.
Studying it
List tasks (write, erase, select, recolour, zoom, undo, export) and walk them with a pen, with the pen present but forbidden, and with no pen at all. Log where people get stuck and whether a substitute control can be found.
Independent variables: commands bound only to pen events, whether the toolbar reveals substitutes when the pen is absent. Dependent measures: task completion, time to find a substitute, abandonments from thinking the function is gone.
Hiding the pen is closer to real loss than asking people not to use it.
Where it stops holding
Capabilities whose content is the channel—pressure calligraphy, tilt side-stroke—can only be approximated without a pen (a width slider); there is no need to fake pressure. A legal signature that requires handwritten ink may be ineligible without a pen, which is a regulatory fact, not a UI omission. A game level that “must use the pen” is an explicit rule and should be stated before entry. On desktop, a mouse can hover, so reachability is easier, but pressure and an eraser end are still missing.
Applying it
- Pair every pen-only entry with a visible non-pen entry; reveal it when the pen is absent rather than burying it under “more”.
- Do not leave a pen cursor that can only fail, or block the only button with “please connect a pen”; a hint is fine, the task must continue.
- Accept shared and classroom configs as completable without a pen.
- How to check: take the pen away and walk the full task list; every step needs a finger or keyboard path. Then check those paths still exist with the pen—reachability adds a path, it does not hide the substitute once the pen is back.