feat(option): custom chars in 'winborder' #33772

Problem: winborder option only supported predefined styles and lacked support for custom border characters.

Solution: implement parsing for comma-separated list format that allows specifying 8 individual border characters (topleft, top, topright, right, botright, bottom, botleft, left).

runtime/doc/news.txt

@@ -229,7 +229,7 @@ OPTIONS
 • 'diffopt' `inline:` configures diff highlighting for changes within a line.
 • 'grepformat' is now a |global-local| option.
 • 'pummaxwidth' sets maximum width for the completion popup menu.
-• 'winborder' "bold" style.
+• 'winborder' "bold" style, custom border style.
 • |g:clipboard| accepts a string name to force any builtin clipboard tool.
 • 'busy' sets a buffer "busy" status. Indicated in the default statusline.
 

runtime/doc/options.txt

@@ -7410,6 +7410,10 @@ A jump table for the options with a short description can be found at |Q_op|.
 	- "shadow": Drop shadow effect, by blending with the background.
 	- "single": Single-line box.
 	- "solid": Adds padding by a single whitespace cell.
+	- custom: comma-separated list of exactly 8 characters in clockwise
+	  order starting from topleft. Example: >lua
+	     vim.o.winborder='+,-,+,|,+,-,+,|'
+<
 
 							*'window'* *'wi'*
 'window' 'wi'		number	(default screen height - 1)

runtime/lua/vim/_meta/options.lua

@@ -8117,8 +8117,15 @@ vim.wo.winbl = vim.wo.winblend
 --- - "shadow": Drop shadow effect, by blending with the background.
 --- - "single": Single-line box.
 --- - "solid": Adds padding by a single whitespace cell.
+--- - custom: comma-separated list of exactly 8 characters in clockwise
+---   order starting from topleft. Example:
 ---
---- @type ''|'double'|'single'|'shadow'|'rounded'|'solid'|'bold'|'none'
+--- ```lua
+---      vim.o.winborder='+,-,+,`,+,-,+,`'
+--- ```
+---
+---
+--- @type string
 vim.o.winborder = ""
 vim.go.winborder = vim.o.winborder
 

src/nvim/api/win_config.c

@@ -1064,6 +1064,52 @@ static void generate_api_error(win_T *wp, const char *attribute, Error *err)
   }
 }
 
+/// Parses a border style name or custom (comma-separated) style.
+bool parse_winborder(WinConfig *fconfig, Error *err)
+{
+  if (!fconfig) {
+    return false;
+  }
+  Object style = OBJECT_INIT;
+
+  if (strchr(p_winborder, ',')) {
+    Array border_chars = ARRAY_DICT_INIT;
+    char *p = p_winborder;
+    char part[MAX_SCHAR_SIZE] = { 0 };
+    int count = 0;
+
+    while (*p != NUL) {
+      if (count >= 8) {
+        api_free_array(border_chars);
+        return false;
+      }
+
+      size_t part_len = copy_option_part(&p, part, sizeof(part), ",");
+      if (part_len == 0 || part[0] == NUL) {
+        api_free_array(border_chars);
+        return false;
+      }
+
+      String str = cstr_to_string(part);
+      ADD(border_chars, STRING_OBJ(str));
+      count++;
+    }
+
+    if (count != 8) {
+      api_free_array(border_chars);
+      return false;
+    }
+
+    style = ARRAY_OBJ(border_chars);
+  } else {
+    style = CSTR_TO_OBJ(p_winborder);
+  }
+
+  parse_border_style(style, fconfig, err);
+  api_free_object(style);
+  return !ERROR_SET(err);
+}
+
 static bool parse_win_config(win_T *wp, Dict(win_config) *config, WinConfig *fconfig, bool reconf,
                              Error *err)
 {
@@ -1297,15 +1343,16 @@ static bool parse_win_config(win_T *wp, Dict(win_config) *config, WinConfig *fco
       goto fail;
     }
     border_style = config->border;
-  } else if (*p_winborder != NUL && (wp == NULL || !wp->w_floating)) {
-    border_style = CSTR_AS_OBJ(p_winborder);
-  }
     if (border_style.type != kObjectTypeNil) {
       parse_border_style(border_style, fconfig, err);
       if (ERROR_SET(err)) {
         goto fail;
       }
     }
+  } else if (*p_winborder != NUL && (wp == NULL || !wp->w_floating)
+             && !parse_winborder(fconfig, err)) {
+    goto fail;
+  }
 
   if (HAS_KEY_X(config, style)) {
     if (config->style.data[0] == NUL) {

src/nvim/api/win_config.h

@@ -4,6 +4,7 @@
 
 #include "nvim/api/keysets_defs.h"  // IWYU pragma: keep
 #include "nvim/api/private/defs.h"  // IWYU pragma: keep
+#include "nvim/buffer_defs.h"  // IWYU pragma: keep
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "api/win_config.h.generated.h"

src/nvim/options.lua

@@ -10431,6 +10431,9 @@ local options = {
       type = 'number',
     },
     {
+      full_name = 'winborder',
+      scope = { 'global' },
+      cb = 'did_set_winborder',
       defaults = { if_true = '' },
       values = { '', 'double', 'single', 'shadow', 'rounded', 'solid', 'bold', 'none' },
       desc = [=[
@@ -10443,11 +10446,14 @@ local options = {
         - "shadow": Drop shadow effect, by blending with the background.
         - "single": Single-line box.
         - "solid": Adds padding by a single whitespace cell.
+        - custom: comma-separated list of exactly 8 characters in clockwise
+          order starting from topleft. Example: >lua
+             vim.o.winborder='+,-,+,|,+,-,+,|'
+        <
       ]=],
-      full_name = 'winborder',
-      scope = { 'global' },
       short_desc = N_('border of floating window'),
       type = 'string',
+      list = 'onecomma',
       varname = 'p_winborder',
     },
     {

src/nvim/optionstr.c

@@ -4,6 +4,7 @@
 #include <string.h>
 
 #include "nvim/api/private/defs.h"
+#include "nvim/api/win_config.h"
 #include "nvim/ascii_defs.h"
 #include "nvim/autocmd.h"
 #include "nvim/buffer_defs.h"
@@ -2101,6 +2102,19 @@ const char *did_set_winbar(optset_T *args)
   return did_set_statustabline_rulerformat(args, false, false);
 }
 
+/// The 'winborder' option is changed.
+const char *did_set_winborder(optset_T *args)
+{
+  WinConfig fconfig = WIN_CONFIG_INIT;
+  Error err = ERROR_INIT;
+  if (!parse_winborder(&fconfig, &err)) {
+    api_clear_error(&err);
+    return e_invarg;
+  }
+  api_clear_error(&err);
+  return NULL;
+}
+
 /// The 'winhighlight' option is changed.
 const char *did_set_winhighlight(optset_T *args)
 {

test/functional/ui/float_spec.lua

@@ -10938,7 +10938,16 @@ describe('float window', function()
       winid = api.nvim_open_win(buf, false, config)
       eq('┏', api.nvim_win_get_config(winid).border[1])
 
-      -- it is currently not supported.
+      command([[set winborder=+,-,+,\|,+,-,+,\|]])
+      winid = api.nvim_open_win(buf, false, config)
+      eq('+', api.nvim_win_get_config(winid).border[1])
+
+      command([[set winborder=●,○,●,○,●,○,●,○]])
+      winid = api.nvim_open_win(buf, false, config)
+      eq('●', api.nvim_win_get_config(winid).border[1])
+
+      eq('Vim(set):E474: Invalid argument: winborder=,,', pcall_err(command, 'set winborder=,,'))
+      eq('Vim(set):E474: Invalid argument: winborder=+,-,+,|,+,-,+,', pcall_err(command, [[set winborder=+,-,+,\|,+,-,+,]]))
       eq('Vim(set):E474: Invalid argument: winborder=custom', pcall_err(command, 'set winborder=custom'))
     end)
   end