Fill

a stroke that can't stop halfway.

drag the marker a direction; it runs until it hits a wall. fill the sheet without sealing yourself off

A stroke here can't stop where you want, only where the wall is, so your first move is hardest to take back. Plan backward from the corner you want to end in: an early stroke can seal off paper you can never reach.

Concepts: planning, constraint, commitment.

An interactive you play with one thumb, on Wundr.