Steps to repro:

1. Pin Unity icon tray to OSX Dock panel (right click -> Options -> Keep in Dock);
2. Open Unity on OSX;
3. Open existing project;
4. Select File -> New Project;
5. Enter project name and click Create Project;
6. Observe tray icon.

Actual result:
Unity opens another tray icon for new project created in opened Editor; previous pinned icon is unclickable. See attached video.

Regression introduced in Unity 5.4.0b5.

Expected result:
New project should always open in the same tray icon in Dock.

Notes:
- Reproducible in: 5.4.0b6, 5.4.0b5;
- Not reproducible in 5.4.0b4;
- Not reproducible on Windows 10;

- The issue may not reproduce every time. In case it doesn't try following:
1) Initially open project which has some changes (scene, gameobjects etc) or which has been worked for a long time.
2) Modify something, save scene/project and then create new one - the issue should reproduce then.
3) Collapse/expand Editor window.