To reproduce, choose Debug > Use ATSU For All Text, select a single word from the middle of a sentence on this page, and then try to drag the selected text. Although drag and drop behaves as expected, there's no drag image, and with a debug build you get this in the console:
ERROR: ATSUDrawText() failed(-8792)
This is a regression from r14297 (fix for bug 8845).
Created attachment 10889 [details]
This invokes the same code that is used for the drag image, so it can be used for an automated pixel test.
This is very bad but should be easy to fix.
Created attachment 12038 [details]
Comment on attachment 12038 [details]
Landed in r18457.