The “Toggle Preview” button is always buggy for me — the first two or three (sometimes four or five) clicks don’t work, and only if I mouse away and back, pause until I see the tooltip, then slooooowly click without moving the mouse (!), does it work.
The only time this isn’t the case is (I think) when I’ve first started up the Designer and haven’t made any pages to the page/site yet. Then it seems to work fine.
True in Safari and Chrome.