Dragging a clip in the timeline and releasing drag outside of the timeline window freezes clip.

Repro:
1- Open project in attachment.
2- Open scene 'TestScene'.
3- Open timeline and select game object 'New Timeline'.
4- In timeline, drag clip 'ControlPlayableAsset' and release mouse outside of timeline window.

You'll notice that you can't drag the clip anymore. You can unfreeze the timeline by dragging the playhead and change time.

Reproducible: 2017.2.0b9, 2017.3.0 (e36fc2cf0775)
Not reproducible in 2017.1.0f3 with my specific repro, but the problem is also reproducible with other tracks.