> I tried applying it via git apply - which failed.

Sorry.  My mailer probably mangled it.  I'll attach newer versions.

 > After applying your
 > patch manually, I still does not work - the testcase results in a
 > hanging emacs.  I might have screwed the application of the patch up,
 > though.  Can anyone confirm my result?

I can't test emacsclient well on my system (Windows XP).  Also, here
`frame-list' seems broken.  It's OK with one frame but for > 1 frame
evaluating (frame-list) can sometimes return nil.  I have to check
what's going on here, IIUC it takes some time (at least a couple of
seconds) for a frame to show up on Vframe_list.

In any case the patch I attached now tries to

(1) Check whether the frame passed to next_frame is in Vframe_list and
     quit if it isn't.

(2) Put a maximum of 100 frames investigated on the loop in next_frame
     so there should be no endless looping otherwise.

martin