I first noticed this issue in Firefox when I was unable to drag tabs without them popping out into a window of their own, but I've since reproduced in the Nemo file manager as well (dragging and dropping files fails to produce any results). Drag and drop does work in Dolphin, which is written using Qt rather than GTK, and drag and drop functions as normal in both Firefox and Nemo in kwin so it seems some particularity of GTK's XDnD implementation isn't being accounted for by worm.

All I've been able to find about peculiarities in GTK's drag and drop so far is this comment in i3's source code but I'm not certain it's relevant.