I run steam on Mac OSX and recently purchased the sam and max complete pack. I downloaded Ice Station Santa, and played it with no problems, getting to Santa's workshop, then back to the main street, and got the first horseman doll. Then I saved, quit, and turned off my computer.
Today, I wanted to play some more, so I brought up steam, clicked "play"and suddenly my screen became... "fragmented" splitting up and mixing around. I could bring up the shift-tab screen, and use that, but on hitting shift-tab again it only added itself as overlay onto the glitched screen. Well, after being forced to do a hard restart. I "deleted local content" from steam, a few times, and that didn't help the problem. and had to hard restart each time.
So I went into my computer and deleted the "telltale games" folder in application support, then I deleted the application shortcut. And this fixed that problem and created a new one. Now when I bring up the game, I see nothing. There's a black screen, and behind it I hear the opening cut scene. And after this plays out, I can hear that my clicking things has the expected result. But I see nothing, cannot ctrl-tab out, and am forced to hard-restart again.
SO, in order to tell the extent of the problem, I downloaded the next game in the series, Moai Better blues, and it had the same black screen problem. But, portal worked fine, so I know that this is a problem with games from telltale games.
the glitched screen looked kinda like this: