The physics
Gravity gives the dart and the monkey the same acceleration g downward. So their relative acceleration is zero.
a(dart) − a(monkey) = g − g = 0
Relative to the monkey, the dart moves in a straight line at constant velocity. Aim that line at the monkey, and the dart must meet it. It only has to get there before the monkey lands.
meeting time t = L / u hit height = H − ½ g t²
A camera that falls freely sees exactly this: no gravity, straight-line motion.