In Windows an EditorWindow floats on top of the main Unity window, which makes sense, but on a Mac this isn’t the case. This makes EditorWindows much harder to work with on a Mac. I swear I’ve seen this problem “fixed” numerous times in various Unity release notes, but can’t figure out how to fix it myself!
I just discovered a couple of little tricks which help to workaround this annoying issue:
- You can drag and drop from a background window to the focused window using the right mouse button without losing focus.
- You can right-click (or
Commandand Left Click) buttons in background windows, again without losing focus.
Also, the problem doesn’t occur when running Unity in full screen mode on OS X Lion+ (provided that editor windows are shown after entering full screen mode).
These are all workarounds, it would be really nice if Unity addressed this issue.
@Alex-Chouls - Not sure if this is helpful since it is no longer ‘dockable’- but it DOES keep your EditorWindow on top even when it loses focus:
Hope this helps. 
Ken