For example, I already own several games in the "Sega Genesis Classics" compilation. I want to know which one's I don't own.

To do that I currently have to,

1. Click "Contains (53)"
2. Click the box art for the 1st game.
3. Look for the collection icon (or lack thereof) on the new page.
4. Press back on my browser
5. Repeat steps 1-4 for all 53 games.

Furthermore, opening the games in new tabs doesn't help, because I still have to re-click "Contains (53)" for each and every game.

The next release will include the standard icon (as a hyperlink) to indicate if a game is in your collection within the compilation popover. I've also modified the various popovers on this page so they don't close if you click inside of them (to make opening in new tabs a nicer experience.)