The button collapse button gets hidden when hovering it. This is not a problem for the expanding one. Might be related with a very long lists of feeds within of them and maybe their title length or something. It isn’t a problem on other folders. Only on the left botton it’s still possible to click the button. Browser is latest Google Chrome on latest Mac OS X.
It’s a Chrome bug, unfortunately. If it wasn’t, there would be something I could do about it.
Okay, so we’ll hope this get fixed in WebKit/Blink soon. :-/
I guess I’m not the only one who thought to report this with an animated GIF 
Sam, any idea roughly what the bug is or where it lies in the source? Perhaps a Chrome-specific fix is possible.
I have the same issue. Using Safari.
The problem is that I don’t get it in latest Chrome or Webkit nightly (Safari). I’d be more than happy to develop a fix immediately if I could only reproduce it.
Well, it doesn’t happen for all folders, only specific ones. I suspect the folder must contain a lot of items or items with a specific (long?) title or something. Just tested in the current Chrome, still exists.
It happens for all of mine. It happens on folders I imported from Google Reader, ones I just created, ones with lots of items, ones with few items, ones with long titles and ones with short titles. I have a macbook pro and OS X 10.8.3 with Safari. I don’t know what webkit nightly is. If I use it, I’m not aware that I do.
Also, I installed the app on an Iphone 4s using iOS 6.1.3. The folders collapse just fine there, but not in the web app. Collapsing on my phone does not sync with the web app. Haven’t tried it on my IPad yet.