menu bar on the left pops up and disappears

When I'm using Safari there pops up and disappears a grey menu on the left, very quick. I don't know how to disable it. On firefox it's not visible. I made a short movie of it to show the problem: https://dl.dropboxusercontent.com/u/50395194/2016-04-21%2013.41.09.mov Does anybody have a solution for this? It's is the last problem before my site is finished. Link to website: www.zwartopwitdesign.nl Thank you in advance! Kind regards, Centaine