I had an identical issue with a site I made a couple months ago and the problem was, in fact, both css and text-length.
Based on Valve's apparent strategy with SteamOS, leaving that link phrased as it is might be useful. As such, I'd suggest just shrinking the search box.
The store layout appears to have changed and I'm not seeing this problem. Is it still happening for you?
The layout definitely seems to have changed and now has a "For You" menu which seems to have replaced the "Recommended" button and the "Linux + Steam OS" button is gone. You can still get to the Linux specific games through the "Games" drop down menu.
There is now lots of space next to the search box so it no longer drops down to the next line. So this issue is no longer relevant.
Nothing extracted yet.
The top navigation bar of the store page now shows "Linux + Steam OS" instead of just "Linux". This by default on my current system seems to push the search box off to the next line. I assume this is due to the extra text and not some CSS problem.
Suggestions:
-Resize the search box to fill the available space instead of a fixed size.
-Pass the distro version from the Steam Client in the HTTP user-agent header then the site can detect for SteamOS or if not Linux.
-Uses something generic across all platforms, e.g. "My Platform", "Current OS", etc.