A screenshot of the Test application in action is attached to this report
called combos_yosemite.jpg.

Normal, non-editable comboboxes look good (1st combobox at
combos_yosemite.jpg), but you can see four problems with the rendering of
editable combo boxes (2nd combobox and 3rd combobox in all frames at
combos_yosemite.jpg):

problem 1: If an editable combobox is not selected, there is a one-pixel
step/displacement between the text field and the button. See 1st frame, 2nd
combobox at combos_yosemite.jpg and 2nd combobox at expected_vs_actual.png
Also if a custom editable combobox is not selected, there is a one-pixel
step/displacement between the text field and the button. See 1st frame, 3rd
combobox at combox_yosemite.jpg and 2nd combobox at expected_vs_actual.png

problem 2: If an editable compbobox component has been selected, the focus
ring around it has a similar step/displacement. See 2nd frame, 2nd combobox
at combos_yosemite.jpg

problem 3: If a custom combobox editor has been used, the focus ring around
the component is not around the entire component. See 3rd frame, 3rd combobox
at combox_yosemite.jpg

problem 4: note that there is not only the step, but also two darker pixels
in the 2nd combobox in expected_vs_actual.png