The Timeline play button stops responding/does nothing after un-linking the Animation Window.

repro:
- create a Timeline a with an Animation track & clip.
- double-click the clip to open the Animation Window in a linked mode
- click the linked mode mutton in the Animation Window to un-link the animation clip
- click the play button in the Timeline window...
bug: play button no longer responds, it does nothing. disabling, then re-enabling Preview does not fix this.

workaround: clear (change selection) and reload (re-select) the Timeline in the Timeline window... play button works again.