This just popped up for me as well starting with r488 on a Mac with Safari. I switched to Chrome and sometimes have the issue but it seems to be less often. It seems to be more prevalent on the Configuration screens while the control screens seem to do it less often. If I resize the window I can see the buttons for a split second when the window refreshes and then they disappear again. Using an iPad seems to work more reliably (odd since it's basically the same webkit code base) but it also exhibits the issue. I haven't tried Firefox basically because I can't bring myself to install that 2nd rate browser, but I get that it's renderer isn't webkit based whereas Safari and Chrome are.
My guess would be that a javascript issue that should be easy to fix.
