The system's pop-up CDEF has a problem that is short-circuited if a 'dctb' is present. The popup menu in the dialog that will navigate through folders is mis-positioned far to the right if you use CustomGetFilePreview when the "Show Preview" check box is unchecked. This happens when the DLOG/DITL resource is not associated with a 'dctb' resource, in other words, when the default color table is specified with ResEdit.
The workaround is to create a 'dctb' for the dialog.