HighlightError.hs

import XMonad.ManageHook ((-->))

hands :: Int
hands = 5 + undefined

complaint :: Int -> String
complaint _ = "Lots of things are red now for no reason. Also, strings aren't highlighted right."

Workaround.hs

-- With this workaround, the file is highlighted correctly.
import XMonad.ManageHook ((-->
                         ))


-- Interestingly, when (-->) is used as an operator, there's no problem:
example :: a
example = hands --> complaint -- "complaint" doesn't have comment markup!
-- I wonder if it's just the parentheses?
example' :: a
example' = (-->) hands complaint

-- Since both of these cases are free from the problem, I think we can conclude the error is somewhere in the import list processing.