Just to give a clean, up-to-date answer to this:
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Select an interface object, then “Editor | Arrange | Send to back/front/etc”, OR
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Select the window object, then click on ‘Window’ above the interface editor and select the objects it contains
- This pic shows where to click:
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Or you can expand the object browser, use the disclosure triangle on the window object, and select from the list there: http://i.imgur.com/041xz.png
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If the option to send to front/back is greyed out, try double-clicking the object to select it.
- Thanks to Bryce Thomas for this suggestion in the comments
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Alternatively, the objects are listed from back to front in the order that they appear in the object browser list.
- Drag an object in the list to the last slot in the list to bring it to the front, for example.