Two line segments meet end to end. The bottom one is nudged left or right. Say which way. Every hit makes the nudge smaller, every miss makes it bigger, until the test finds the smallest shift your eye can call.
Round 1
Which way did the bottom line move?
First, guess: smallest shift I can see
Line width
Trial 0 of 40 maxCurrent shift: -Keys: ← →
Your threshold
?px
Finish a round (8 reversals) to measure it.
Trial log
#
Shift (px)
Your answer
Result
What that shift looks like in a UI
The same card twice. On the right the icon sits a few pixels off the title's left edge. Drag the shift and see where you stop noticing.