Problem:
`Completor:accept()` applies the item against the range the server
answered with. Characters typed after the request but before accepting
lie inside neither that range nor the replacement, so they are left
behind by the insertion: typing `a` after `foob` and accepting `foobar`
within the 200 ms debounce yields `foobara`.
Solution:
Grow the item's range over the characters typed since the request, so
that `accept()` replaces them. Only while the candidate still matches,
decided by the longest common prefix `show()` already computes, so input
that contradicts it is left alone. Snippet items are unaffected, since
`accept()` ignores the range for them.
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