fix(ui): always use stl/stlnc fillchars when drawing statusline (#25267)

runtime/doc/options.txt

@@ -2465,8 +2465,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 	and the value of that item:
 
 	  item		default		Used for ~
-	  stl		' ' or '^'	statusline of the current window
-	  stlnc		' ' or '='	statusline of the non-current windows
+	  stl		' '		statusline of the current window
+	  stlnc		' '		statusline of the non-current windows
 	  wbr		' '		window bar
 	  horiz		'─' or '-'	horizontal separators |:split|
 	  horizup	'┴' or '-'	upwards facing horizontal separator
@@ -2485,9 +2485,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	  eob		'~'		empty lines at the end of a buffer
 	  lastline	'@'		'display' contains lastline/truncate
 
-	Any one that is omitted will fall back to the default.  For "stl" and
-	"stlnc" the space will be used when there is highlighting, '^' or '='
-	otherwise.
+	Any one that is omitted will fall back to the default.
 
 	Note that "horiz", "horizup", "horizdown", "vertleft", "vertright" and
 	"verthoriz" are only used when 'laststatus' is 3, since only vertical

runtime/doc/syntax.txt

@@ -5322,8 +5322,6 @@ SpellRare	Word that is recognized by the spellchecker as one that is
 StatusLine	Status line of current window.
 							*hl-StatusLineNC*
 StatusLineNC	Status lines of not-current windows.
-		Note: If this is equal to "StatusLine", Vim will use "^^^" in
-		the status line of the current window.
 							*hl-TabLine*
 TabLine		Tab pages line, not active tab page label.
 							*hl-TabLineFill*

runtime/doc/vim_diff.txt

@@ -534,6 +534,10 @@ Vimscript compatibility:
   `shell_error` does not alias to |v:shell_error|
   `this_session` does not alias to |v:this_session|
 
+UI/Display:
+  'statusline' always uses the "stl" and "stlnc" fields of 'fillchars', even
+  if they are the same and |hl-StatusLine| and |hl-StatusLineNC| are equal.
+
 Working directory (Vim implemented some of these after Nvim):
 - |DirChanged| and |DirChangedPre| can be triggered when switching to another
   window or tab.

runtime/lua/vim/_meta/options.lua

@@ -2181,8 +2181,8 @@ vim.bo.ft = vim.bo.filetype
 --- and the value of that item:
 ---
 ---   item		default		Used for ~
----   stl		' ' or '^'	statusline of the current window
----   stlnc		' ' or '='	statusline of the non-current windows
+---   stl		' '		statusline of the current window
+---   stlnc		' '		statusline of the non-current windows
 ---   wbr		' '		window bar
 ---   horiz		'─' or '-'	horizontal separators `:split`
 ---   horizup	'┴' or '-'	upwards facing horizontal separator
@@ -2201,9 +2201,7 @@ vim.bo.ft = vim.bo.filetype
 ---   eob		'~'		empty lines at the end of a buffer
 ---   lastline	'@'		'display' contains lastline/truncate
 ---
---- Any one that is omitted will fall back to the default.  For "stl" and
---- "stlnc" the space will be used when there is highlighting, '^' or '='
---- otherwise.
+--- Any one that is omitted will fall back to the default.
 ---
 --- Note that "horiz", "horizup", "horizdown", "vertleft", "vertright" and
 --- "verthoriz" are only used when 'laststatus' is 3, since only vertical

src/nvim/options.lua

@@ -2790,8 +2790,8 @@ return {
         and the value of that item:
 
           item		default		Used for ~
-          stl		' ' or '^'	statusline of the current window
-          stlnc		' ' or '='	statusline of the non-current windows
+          stl		' '		statusline of the current window
+          stlnc		' '		statusline of the non-current windows
           wbr		' '		window bar
           horiz		'─' or '-'	horizontal separators |:split|
           horizup	'┴' or '-'	upwards facing horizontal separator
@@ -2810,9 +2810,7 @@ return {
           eob		'~'		empty lines at the end of a buffer
           lastline	'@'		'display' contains lastline/truncate
 
-        Any one that is omitted will fall back to the default.  For "stl" and
-        "stlnc" the space will be used when there is highlighting, '^' or '='
-        otherwise.
+        Any one that is omitted will fall back to the default.
 
         Note that "horiz", "horizup", "horizdown", "vertleft", "vertright" and
         "verthoriz" are only used when 'laststatus' is 3, since only vertical

src/nvim/statusline.c

@@ -637,18 +637,7 @@ int fillchar_status(int *attr, win_T *wp)
     *attr = win_hl_attr(wp, HLF_SNC);
     fill = wp->w_p_fcs_chars.stlnc;
   }
-  // Use fill when there is highlighting, and highlighting of current
-  // window differs, or the fillchars differ, or this is not the
-  // current window
-  if (*attr != 0 && ((win_hl_attr(wp, HLF_S) != win_hl_attr(wp, HLF_SNC)
-                      || !is_curwin || ONE_WINDOW)
-                     || (wp->w_p_fcs_chars.stl != wp->w_p_fcs_chars.stlnc))) {
-    return fill;
-  }
-  if (is_curwin) {
-    return '^';
-  }
-  return '=';
+  return fill;
 }
 
 /// Redraw the status line according to 'statusline' and take care of any

test/functional/ui/statusline_spec.lua

@@ -799,3 +799,21 @@ it("shows correct ruler in cmdline with no statusline", function()
                    3longlonglong  |
   ]]
 end)
+
+it('uses "stl" and "stlnc" fillchars even if they are the same #19803', function()
+  clear()
+  local screen = Screen.new(53, 4)
+  screen:attach()
+  screen:set_default_attr_ids({
+    [1] = {bold = true, foreground = Screen.colors.Blue},  -- NonText
+  })
+  command('hi clear StatusLine')
+  command('hi clear StatusLineNC')
+  command('vsplit')
+  screen:expect{grid=[[
+    ^                          │                          |
+    {1:~                         }│{1:~                         }|
+    [No Name]                  [No Name]                 |
+                                                         |
+  ]]}
+end)