display problem Audacity 2.1.1 on Yosemite (Mac osx 10.10.5)

Hi !
I recently installed Audacity 2.1.1 on my macbook (Yosemite / mac os x 10.10.5). It seems to work OK, except on thing : I have an annoying display problem. Everytime I open a track or project into Audacity, the “close” / “minimize” / “enlarge” buttons don’t appear, so I cannot close the window by clicking on the red cross for example. I have to go into the menu to close the project, which is quite long and bothering.
Has anyone heard about this problem before, or know what I could do about it ?
Thank you !

Try Window > Zoom.