I see this on FCPX and I wonder if others are experiencing this, too.
I'm editing along happily in my timeline deleting excess material. Occasionally I delete a section and when I go to play it, nothing happens. The viewer displays the checkered bar across the top like when you click on a project in the browser. In order to "wake" X back up, I have to click into the browser area (anywhere) and then back into the timeline window and now all is fine.
This is intermittent and doesn't happen every time I do it; however, it does happen multiple times as I work through the timeline. So it's not an occasional, random thing. I have seen this behavior on more than one machine - old and new. And on dual and single screen configurations.
FWIW - if I play and delete while playing on-the-fly this doesn't happen. Only with a standard mark-in-out-delete procedure. Thoughts?