>> You're right.  I thought this was happening entirely inside Xlib, and 
>> indeed it requires some cooperation of the program.  It seems that this 
>> cooperation is minimal, though: IIUC, the client should just 
>> immediately discard the events when XFilterEvent returns True, that is, 
>> until the composed character is delivered.
>
> How do you think ``the composed character is delivered''?
>

Like any other character, with a KeyPress event, but through 
XmbLookupString (its first occurrence in handle_one_xevent).  AFAIU what 
happens is this:

Compose -> KeyPress event, keysym Multi_key, with XFilterEvent True
_       -> KeyPress event, keysym underscore, with XFilterEvent True
>       -> KeyPress event, keysym greater, with XFilterEvent True

after which a KeyPress event, keysym U2265, with XFilterEvent False, and 
for which XmbLookupString returns "≥", is delivered to the client.

What am I missing?