fix(statusline): scope truncation bookkeeping

Limit the default truncation item to the current recursion range so
nested `nvim_eval_statusline()` calls don't reuse stale `stl_items`
pointers. Add a functional regression that evaluates a Lua statusline
helper which forces truncation to ensure the nested scenario stays
stable.

AI-Assist: OpenAI ChatGPT

Fixes #36616

src/nvim/statusline.c

@@ -1902,7 +1902,7 @@ stcsign:
   int width = vim_strsize(out);
   if (maxwidth > 0 && width > maxwidth && (!stcp || width > MAX_STCWIDTH)) {
     // Result is too long, must truncate somewhere.
-    int item_idx = 0;
+    int item_idx = evalstart;
     char *trunc_p;
 
     // If there are no items, truncate from beginning
@@ -1912,8 +1912,7 @@ stcsign:
       // Otherwise, look for the truncation item
     } else {
       // Default to truncating at the first item
-      trunc_p = stl_items[0].start;
-      item_idx = 0;
+      trunc_p = stl_items[item_idx].start;
 
       for (int i = evalstart; i < itemcnt + evalstart; i++) {
         if (stl_items[i].type == Trunc) {

test/functional/ui/statusline_spec.lua

@@ -802,6 +802,28 @@ describe('statusline', function()
                                               |
     ]])
   end)
+
+  it('truncation inside nested nvim_eval_statusline does not crash #36616', function()
+    exec_lua(function()
+      function _G.statusline_truncating()
+        local win = vim.api.nvim_get_current_win()
+        local res = vim.api.nvim_eval_statusline('%f', { winid = win, maxwidth = 5 })
+        return res.str
+      end
+      vim.o.laststatus = 2
+      vim.o.statusline = '%#Special#B:%{%v:lua.statusline_truncating()%}'
+    end)
+    local truncated = exec_lua(function()
+      return vim.api.nvim_eval_statusline('%f', { maxwidth = 5 }).str
+    end)
+    local rendered = exec_lua(function()
+      return vim.api.nvim_eval_statusline(
+        vim.o.statusline,
+        { winid = vim.api.nvim_get_current_win() }
+      ).str
+    end)
+    eq('B:' .. truncated, rendered)
+  end)
 end)
 
 describe('default statusline', function()