Reproduction Steps:
1. Create Project
2. Create UI/Image object
3. Place Editor on Retina display
4. Click Color Picker (little pipette icon)
5. Move it around the screen
6. Notice that when it moves - colours change
7. Click on some icons or windows outside of editor (still same screen) (for example, dock icons)
8. Notice that colour is changed and (sometimes) editor gets unfocused
9. Select Color Picker again and move to other screen
10. Notice that it stops tracking colour
11. Click somewhere on that screen
12. Notice it does not change colour and loses focus of editor
13. Select Color Picker, move to other screen and back and click
14. Notice colour change
15. Move Editor to non-retina screen
16. Do the same
17. Notice that clicking on other screen changes colour and loses focus of editor

Reproduced on versions: Unity 5.3.6p4, 5.4.1f1, 5.5.0b1, 5.5.0b2
Reproduced on OSX 10.11.6
Did not reproduce on Windows 10