feat(diagnostic): vim.diagnostic.setqflist improvements #30868

1. Use the new "u" action to update the quickfix list so we don't lose
   our position in the quickfix list when updating it.
2. Rather than creating a new quickfix list each time, update the
   exiting one if we've already created one.

runtime/doc/news.txt

@@ -303,6 +303,8 @@ UI
   |hl-PmenuSel| and |hl-PmenuMatch| both inherit from |hl-Pmenu|, and
   |hl-PmenuMatchSel| inherits highlights from both |hl-PmenuSel| and
   |hl-PmenuMatch|.
+• |vim.diagnostic.setqflist()| updates existing diagnostics quickfix list if one
+  exists.
 
 • |ui-messages| content chunks now also contain the highlight group ID.
 

runtime/lua/vim/diagnostic.lua

@@ -2,6 +2,8 @@ local api, if_nil = vim.api, vim.F.if_nil
 
 local M = {}
 
+local _qf_id = nil
+
 --- [diagnostic-structure]()
 ---
 --- Diagnostics use the same indexing as the rest of the Nvim API (i.e. 0-based
@@ -848,9 +850,24 @@ local function set_list(loclist, opts)
   local diagnostics = get_diagnostics(bufnr, opts --[[@as vim.diagnostic.GetOpts]], false)
   local items = M.toqflist(diagnostics)
   if loclist then
-    vim.fn.setloclist(winnr, {}, ' ', { title = title, items = items })
+    vim.fn.setloclist(winnr, {}, 'u', { title = title, items = items })
   else
-    vim.fn.setqflist({}, ' ', { title = title, items = items })
+    -- Check if the diagnostics quickfix list no longer exists.
+    if _qf_id and vim.fn.getqflist({ id = _qf_id }).id == 0 then
+      _qf_id = nil
+    end
+
+    -- If we already have a diagnostics quickfix, update it rather than creating a new one.
+    -- This avoids polluting the finite set of quickfix lists, and preserves the currently selected
+    -- entry.
+    vim.fn.setqflist({}, _qf_id and 'u' or ' ', {
+      title = title,
+      items = items,
+      id = _qf_id,
+    })
+
+    -- Get the id of the newly created quickfix list.
+    _qf_id = vim.fn.getqflist({ id = 0 }).id
   end
   if open then
     api.nvim_command(loclist and 'lwindow' or 'botright cwindow')