Created attachment 36188 [details]
Image of collected screenshots showing misbehavior of U+0331 with XeTeX.

Using XeTeX, which should be properly supporting mark positioning, the character U+0331 Combining Macron Below is not rendered correctly. The attached screenshot shows the sequence U+0078 Latin Small Letter X, U+0331 Combining Macron Below, and U+02BC Modifier Letter Apostrophe as produced by XeTeX (version 3.1415926-2.2-0.9995.2 (TeX Live 2009)). The U+02BC Modifier Letter Apostrophe behaves correctly in the other screenshot which shows only the sequence U+0078 U+0331 without the U+0331 Combining Macron Below.

Other fonts which support U+0331 display the problematic sequence correctly, as shown by the third screenshot which uses Bitstream/Apple’s Courier (v. 6.1d5e1), although note the incorrect difference between the height of U+0331 versus the diacritic appearing as part of the precombined U+1E34 Latin Capital Letter K with Line Below.