reproducible language server desync

[fpvandoorn]

Prerequisites

Please put an X between the brackets as you perform the following steps:

• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
  (You can also use the settings there to switch to “Lean nightly”)

Description

Step 1: copy-paste the following in an empty file:

structure Signature where
  /-- a docstring -/
  Sort : Type

This will give an expected error, since Sort is a reserved keyword.

Step 2: add the letter s to Sort, so that the line reads Sorts : Type

On Windows, the error unexpected token 'Sort' remains until the file is restarted.

Versions

I can reproduce this on 4.14.0-rc2, 4.13.0-rc3 (on Windows) and on live.lean-lang, so it doesn't seem OS specific.

Impact

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