Eg in gimp's "Save as…" dialogue:

When one tries to tab-complete a path in the "Name:" field, it seems to (sometimes) work properly as long as there are no further suggestions (ie either only one match or none), but once there are more (ie the drop down list with them appears), tab does not circle through them, but 'advances' the keyboard focus to the 'file system' icon which is the leftmost button in the "Save in folder:" field.

If one then 'inverse tab's (<S-Tab>) back and tab-completes once more, the completion works properly — but only for that folder level and until the first ambiguity.

Notes:

Open dialogues appear not affected, perhaps as their "Save in folder:" 'complement' (the one next to the pen&paper icon) is placed above the "Location:" field instead. gtk+ 3.4-git dialogues also seem unaffected.