How to reproduce:
1. Open the user's attached "BugReport" project
2. Open the Scene "SampleScene"
3. In the Hierarchy select the "Vine1" GameObject
4. Open the Animator (Window->Animation->Animator)
5. In the Animator select the transition between "Vine1 OpenToIdle" and "Idle" states
6. In the Inspector convert the preview window to floating (in the preview window top-right corner press the vertical ellipsis and select "Convert to Floating Window")
7. In the floating transition preview window drag the playback marker

Expected results: The playback marker moves to the position it is dragged to, the animation shows the chosen frame
Actual results: The playback marker doesn't move, the animation doesn't show the chosen frame

Reproducible with: 2018.4.29f1, 2019.4.15f1, 2020.1.15f1, 2020.2.0b13, 2021.1.0a7

Notes:
- The playback marker works when the animation is playing
- The playback marker's position is changed when you press anywhere on the playback marker area and pressing play, the Animation will start from the spot where it was selected