The only time a description would be hidden behind a "See full item description" button in a desktop view would be if the seller screwed up and did not update his listings to comply with active content and other listing HTML issues.
The collapsed hidden description display is the default for mobile browser viewing and I believe mobile app display also, and there is no setting to change that.
If you are seeing descriptions hidden on all listings viewed in a desktop browser then something is wrong (or you are somehow picking the 0.001% of listings that are messed up).
What OS/browser/browser version are you using? Post the URL of one of the listings that is a problem.
