After pairing drops, writing input should degrade smoothly to finger touch rather than fail entirely

What it is

When the pen drops, the note in progress must not become a dead canvas. Touch, on-screen controls, and the keyboard should be able to continue at once: coarse drawing, typing, save, undo. That is channel degradation, in the same family as “every function reachable without a pen”: one is the accessory never being there, the other is it was here a moment ago and vanished. Smooth means the current document is not lost and the UI is not stuck in a “please connect a pen” modal.

Why it happens

If an app binds canvas hit-testing to pen event types only, a finger tap after the drop has no listener. Correct dispatch prefers pen events and, when they are missing, accepts touch on the same canvas (perhaps a fat brush, pressure off). The stroke in flight should end on the last real point, not wait for prediction. A modal “a pen is required to continue” also blocks save, so content at the moment of power loss is at risk. Palm rejection should loosen after the pen vanishes, or a resting hand stays suppressed and fingers cannot tap either. Degradation should be visible: the tool changes from a pressure nib to a finger pencil, so people do not think pressure is still live. Reconnect may promote again, but must not steal a contact while a finger is down.

Studying it

Mid-stroke, kill the pen radio or take the pen away. See whether a finger can save, keep smearing, and raise a keyboard. Compare: dead canvas, modal blocking save, automatic acceptance of touch.

Independent variables: whether event dispatch falls back, whether palm rejection loosens, blocking modal present. Dependent measures: whether save works, time from drop to next valid input, accidental touches, conflict when the pen promotes again.

Where it stops holding

A legal signature mid-way may have to be voided rather than continued with a finger; degradation is then save-as-draft and declare incomplete. A game level that requires the pen should pause rather than let a finger finish it. A capacitive stylus already on the touch channel does not make this jump.

Applying it

• The canvas listens to pen and touch; when the pen is absent, touch fires and the brush becomes pressureless.
• A drop must not lock the document behind a modal; save, undo, and keyboard stay tappable.
• After the pen prior is gone, palm rejection uses a looser rule.
• How to check: take the pen away mid-stroke; a finger should draw a fat line and tap save. There should not be an only-“please connect a pen”, uncancellable dialog. When the pen returns, the next stroke may regain pressure, but an in-flight finger drag must not be stolen.

Related

• Same group: C5.17.1 Pairing can drop mid-session · C5.17.2 Low battery must be explicit, not a silent stop · C5.17.3 Magnets reduce loss but do not replace find-my
• Adjacent: C5.07 Division of Labor Between Pen and Finger · C5.04 Palm Rejection
• Search: graceful degradation · stylus fallback · finger ink