This is something I noticed in BitBucket, which I believe runs Stash. I haven't tested it in a personal Stash instance, but my assumption is that the same issue is there.
Mobile browsers (in my case, Safari on iOS 8) are unable to submit comments to commits, and possibly other relevant parts of the application. There is no submit button, and naturally pressing enter / return enters a new line. The attached image is what I see in my browser.