fix(diagnostic): stack _tags hl-groups in a single extmark #38654

Problem:
Diagnostic highlight groups were applied by iterating and calling
`vim.hl.range` for each group individually. That resulted in multiple
extmarks with the same priority being created separately, which does not
allow `DiagnosticUnnecessary` and `DiagnosticDeprecated` with matching
options override `Diagnostic*` styling.

Solution:
Pass the list of hl-groups to `vim.hl.range` so they are applied
together in the correct order.

runtime/lua/vim/diagnostic/_handlers.lua

@@ -280,16 +280,14 @@ function M.underline.show(namespace, bufnr, diagnostics, opts)
       local lines =
         api.nvim_buf_get_lines(diagnostic0.bufnr, diagnostic0.lnum, diagnostic0.lnum + 1, true)
 
-      for _, higroup in ipairs(higroups) do
-        vim.hl.range(
-          bufnr,
-          underline_ns,
-          higroup,
-          { diagnostic0.lnum, math.min(diagnostic0.col, #lines[1] - 1) },
-          { diagnostic0.end_lnum, diagnostic0.end_col },
-          { priority = get_priority(diagnostic0.severity) }
-        )
-      end
+      vim.hl.range(
+        bufnr,
+        underline_ns,
+        higroups,
+        { diagnostic0.lnum, math.min(diagnostic0.col, #lines[1] - 1) },
+        { diagnostic0.end_lnum, diagnostic0.end_col },
+        { priority = get_priority(diagnostic0.severity) }
+      )
     end
 
     save_extmarks(underline_ns, bufnr)

runtime/lua/vim/hl.lua

@@ -40,7 +40,8 @@ M.priorities = {
 ---
 ---@param buf integer Buffer number to apply highlighting to
 ---@param ns integer Namespace to add highlight to
----@param higroup string Highlight group to use for highlighting
+---@param higroup integer|integer[]|string|string[] Highlight group used for the text range.
+--- See the `hl_group` option in |nvim_buf_set_extmark()|.
 ---@param start [integer,integer]|string Start of region as a (line, column) tuple or string accepted by |getpos()|
 ---@param finish [integer,integer]|string End of region as a (line, column) tuple or string accepted by |getpos()|
 ---@param opts? vim.hl.range.Opts