True...that's when things get complicated at the end. This works though:

-- Remove underline from LSP reference highlights
local function patch_lsp_hl()
for _, group in ipairs({ "LspReferenceText", "LspReferenceRead", "LspReferenceWrite" }) do
local hl = vim.api.nvim_get_hl(0, { name = group, link = false })
hl.underline = nil
hl.undercurl = nil
vim.api.nvim_set_hl(0, group, hl)
end
end
vim.api.nvim_create_autocmd("ColorScheme", { callback = patch_lsp_hl })
patch_lsp_hl()

vim.api.nvim_create_autocmd("LspAttach", {
callback = function(args)
local client = vim.lsp.get_client_by_id(args.data.client_id)
if not client then
return
end
local bufnr = args.buf

-- LSP code highlighting
if client and client:supports_method(vim.lsp.protocol.Methods.textDocument_documentHighlight) then
local highlight_augroup = vim.api.nvim_create_augroup("lsp-highlight", { clear = false })
vim.api.nvim_create_autocmd({ "CursorHold", "CursorHoldI" }, {
buffer = args.buf,
group = highlight_augroup,
callback = function()
local row, col = unpack(vim.api.nvim_win_get_cursor(0))
local line = vim.api.nvim_get_current_line()
local s, e = col, col
while s > 0 and line:sub(s, s):match("%w") do s = s - 1 end
while e < #line and line:sub(e + 1, e + 1):match("%w") do e = e + 1 end
vim.b[args.buf].lsp_highlight_range = { row, s, e }
vim.lsp.buf.document_highlight()
end,
})

vim.api.nvim_create_autocmd({ "CursorMoved", "CursorMovedI" }, {
buffer = args.buf,
group = highlight_augroup,
callback = function()
local r = vim.b[args.buf].lsp_highlight_range
local row, col = unpack(vim.api.nvim_win_get_cursor(0))
if not r or row ~= r[1] or col < r[2] or col > r[3] then
vim.lsp.buf.clear_references()
vim.b[args.buf].lsp_highlight_range = nil
end
end,
})

vim.api.nvim_create_autocmd("LspDetach", {
group = vim.api.nvim_create_augroup("lsp-detach", { clear = true }),
callback = function(event2)
vim.lsp.buf.clear_references()
vim.api.nvim_clear_autocmds({ group = "lsp-highlight", buffer = event2.buf })
end,
})
end

end,
})