Crossing
When two lines disappear behind a mask, the mind fills the gap. That fill is often a little bent, a close cousin of the Poggendorff effect, and the imagined crossing drifts. Short stubs make it worse: the eye starts listening to the mask’s edges instead of the true directions.
Tap to place a mark, drag to adjust. Lifting your finger does not submit.
The scoring formula for Crossing is published on How to play, and the constants there are still initial values.