Steps to reproduce:

1. Save a file
2. (When selecting the path) Set the cursor in the middle of the text
3. Try completing something

Notes:
   get_partial_completion_input_state

If there is interest in changing this behaviour, please pull from git://github.com/fjl/conkeror.git

thanks fjl, but... although the original report is not incredibly clear
about this, the issue is actually that get_partial_completion_input_state
inserts the *wrong* separator, not that it should not insert a separator
at all.  for most of the callers of get_partial_completion_input_state, a
space is the appropriate separator, for example, when completing on the
name of a webjump.  but when completing on a file path, one would expect
it to insert the appropriate path component separator (\ or /) depending
on the OS.  if i recall correctly from the last time i was reading through
this part of conkeror, my determination was that we needed a completer
which was aware of the file system structure, in that it could tell the
difference between a file and a directory, insert appropriate separators,
display appropriate error messages, etc, but that to implement such a
completer without huge redundancy, the system will need a little bit of
refactoring.

to put it succinctly, it's not that prefix_completer does the wrong thing, it's that prefix_completer
is the wrong tool for the job, and it's just a stand-in until we have a proper file-path completer.

You're right about that and i was just being naïve. 

Emacs' default prefix completion simply uses all available 
input for completion and jumps to the end of the line.
It also says "No Match" if the input isn't the prefix of an existing file or directory.

While this is a terrible solution for completing the name of a new file (i.e. when saving something)
this may be a better alternative than inserting a space. 

I'll open a new branch and implement that.

This will be dealt with in the refactoring-completers branch, and resolved when that branch is merged.