I just started having the exact same issue. L4D is playing in minimize mode. When I click to maximize it shows for a second and then minimizes again.

Mine did not, to the best of my knowledge, follow any updating, but I can't say for sure. It worked fine one minute, then this.

Can you tell me how/if you resolved?

I am considering going back to a previous restore point to see if that helps. Or reinstalling, but I thought I would check first.

Please let me know.