It seems that the behaviour of the start parameter being missing is
server (or perhaps a specific client) dependent. The specification
clearely says that it should be inserted at the column of the original
request, but the servers clearly expect either for that column to be the
beginning of an identifier or for the client to ignore the spec and
request from that position anyway.
Reading the VSCode code, we see that the 'word' before the cursor is
guessed, and if only if BOTH 'start' AND 'length' are supplied, then
they are used to determine where insertion starts, otherwise the current
'word' is used. Unclear what 'word' means in the specific contexts, but
we're relying on iskeyword.
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