feat(api): relax statusline fillchar width check

Treat fillchar as single-width even if it isn't.

src/nvim/api/vim.c

@@ -2241,7 +2241,7 @@ Array nvim_get_mark(String name, Dictionary opts, Error *err)
 /// - winid: (number) |window-ID| of the window to use as context for statusline.
 /// - maxwidth: (number) Maximum width of statusline.
 /// - fillchar: (string) Character to fill blank spaces in the statusline (see
-/// 'fillchars').
+/// 'fillchars'). Treated as single-width even if it isn't.
 /// - highlights: (boolean) Return highlight information.
 /// - use_tabline: (boolean) Evaluate tabline instead of statusline. When |TRUE|, {winid}
 /// is ignored.
@@ -2277,11 +2277,12 @@ Dictionary nvim_eval_statusline(String str, Dict(eval_statusline) *opts, Error *
 
  if (HAS_KEY(opts->fillchar)) {
  if (opts->fillchar.type != kObjectTypeString || opts->fillchar.data.string.size == 0
- || char2cells(fillchar = utf_ptr2char((char_u *)opts->fillchar.data.string.data)) != 1
- || (size_t)utf_char2len(fillchar) != opts->fillchar.data.string.size) {
- api_set_error(err, kErrorTypeValidation, "fillchar must be a single-width character");
+ || ((size_t)utf_ptr2len((char_u *)opts->fillchar.data.string.data)
+  != opts->fillchar.data.string.size)) {
+ api_set_error(err, kErrorTypeValidation, "fillchar must be a single character");
  return result;
  }
+ fillchar = utf_ptr2char((char_u *)opts->fillchar.data.string.data);
  }
 
  if (HAS_KEY(opts->highlights)) {

test/functional/api/vim_spec.lua

@@ -2621,24 +2621,24 @@ describe('API', function()
  eq({ str = 'a━━━b', width = 5 },
  meths.eval_statusline('a%=b', { fillchar = '━', maxwidth = 5 }))
  end)
- it('rejects double-width fillchar', function()
- eq('fillchar must be a single-width character',
- pcall_err(meths.eval_statusline, '', { fillchar = '哦' }))
+ it('treats double-width fillchar as single-width', function()
+ eq({ str = 'a哦哦哦b', width = 5 },
+ meths.eval_statusline('a%=b', { fillchar = '哦', maxwidth = 5 }))
  end)
- it('rejects control character fillchar', function()
- eq('fillchar must be a single-width character',
- pcall_err(meths.eval_statusline, '', { fillchar = '\a' }))
+ it('treats control character fillchar as single-width', function()
+ eq({ str = 'a\031\031\031b', width = 5 },
+ meths.eval_statusline('a%=b', { fillchar = '\031', maxwidth = 5 }))
  end)
  it('rejects multiple-character fillchar', function()
- eq('fillchar must be a single-width character',
+ eq('fillchar must be a single character',
  pcall_err(meths.eval_statusline, '', { fillchar = 'aa' }))
  end)
  it('rejects empty string fillchar', function()
- eq('fillchar must be a single-width character',
+ eq('fillchar must be a single character',
  pcall_err(meths.eval_statusline, '', { fillchar = '' }))
  end)
  it('rejects non-string fillchar', function()
- eq('fillchar must be a single-width character',
+ eq('fillchar must be a single character',
  pcall_err(meths.eval_statusline, '', { fillchar = 1 }))
  end)
  describe('highlight parsing', function()