Animation glitch when interactively dismissing modal VC with custom presentation

Number:rdar://36771251 Date Originated:
Status:open Resolved:
Product:iOS Product Version:
Classification: Reproducible:always
While implementing an interactively dismissible controller that doesn't cover the entire screen and can be dismissed with a pan gesture, I came across a weird behavior where the animation jumps back to it's initial position and then back to where it is supposed to be. Sample project:

Steps to Reproduce:
1) Launch the app on device (the issue seems less visible/present on the simulator)
2) open the modal controller using the button
3) slowly drag up or down (works in both directions)

Video of issue:

Expected Results:
The animated view controller follows drag movement.

Actual Results:
The animated view controller briefly returns to initial position.

iOS 11

Xcode 9.2, High Sierra 10.13.2


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