House Painter is a coverage puzzle: you move a roller around a house interior and have to paint every surface, with a limited supply and walls that block your route. Painting over the same spot twice wastes paint you'll need elsewhere.
It's the same shape as a classic 'cover every square once' puzzle, dressed as decorating, which makes route planning the actual challenge rather than the painting itself.
Because you painted over an area twice. The supply is calculated for a route that covers each surface exactly once, so retracing your steps is what causes shortfalls.
It's a route-planning puzzle rather than a race, so the constraint is paint rather than time.
Yes — drag to move. Nothing to install.