feat(option): add winborder option

Problem:
There is currently no global option to define the default border style for floating windows. This leads to repetitive code when developers need consistent styling across multiple floating windows.

Solution:
Introduce a global option winborder to specify the default border style for floating windows. When a floating window is created without explicitly specifying a border style, the value of the winborder option will be used. This simplifies configuration and ensures consistency in floating window appearance.

runtime/doc/api.txt

@@ -3238,20 +3238,12 @@ nvim_open_win({buffer}, {enter}, {config})                   *nvim_open_win()*
                       `eob` flag of 'fillchars' to a space char, and clearing
                       the |hl-EndOfBuffer| region in 'winhighlight'.
                   • border: Style of (optional) window border. This can either
-                    be a string or an array. The string values are
+                    be a string or an array. The string values are the same as
-                    • "none": No border (default).
+                    those described in 'winborder'. If it is an array, it
-                    • "single": A single line box.
+                    should have a length of eight or any divisor of eight. The
-                    • "double": A double line box.
+                    array will specify the eight chars building up the border
-                    • "rounded": Like "single", but with rounded corners
+                    in a clockwise fashion starting with the top-left corner.
-                      ("╭" etc.).
+                    As an example, the double box style could be specified as: >
-                    • "solid": Adds padding by a single whitespace cell.
-                    • "shadow": A drop shadow effect by blending with the
-                      background.
-                    • If it is an array, it should have a length of eight or
-                      any divisor of eight. The array will specify the eight
-                      chars building up the border in a clockwise fashion
-                      starting with the top-left corner. As an example, the
-                      double box style could be specified as: >
                      [ "╔", "═" ,"╗", "║", "╝", "═", "╚", "║" ].
 <
                     If the number of chars are less than eight, they will be

runtime/doc/options.txt

@@ -7105,6 +7105,18 @@ A jump table for the options with a short description can be found at |Q_op|.
 
 	UI-dependent. Works best with RGB colors. 'termguicolors'
 
+							*'winborder'*
+'winborder'		string	(default "")
+			global
+	Defines the default border style of floating windows. The default value
+	is empty, which is equivalent to "none". Valid values include:
+	- "none": No border.
+	- "single": A single line box.
+	- "double": A double line box.
+	- "rounded": Like "single", but with rounded corners ("╭" etc.).
+	- "solid": Adds padding by a single whitespace cell.
+	- "shadow": A drop shadow effect by blending with the background.
+
 							*'window'* *'wi'*
 'window' 'wi'		number	(default screen height - 1)
 			global

runtime/lua/vim/_meta/api.lua

@@ -1806,14 +1806,8 @@ function vim.api.nvim_open_term(buffer, opts) end
 ---                  'fillchars' to a space char, and clearing the
 ---                  `hl-EndOfBuffer` region in 'winhighlight'.
 --- - border: Style of (optional) window border. This can either be a string
----   or an array. The string values are
+---   or an array. The string values are the same as those described in 'winborder'.
----   - "none": No border (default).
+---   If it is an array, it should have a length of eight or any divisor of
----   - "single": A single line box.
----   - "double": A double line box.
----   - "rounded": Like "single", but with rounded corners ("╭" etc.).
----   - "solid": Adds padding by a single whitespace cell.
----   - "shadow": A drop shadow effect by blending with the background.
----   - If it is an array, it should have a length of eight or any divisor of
 ---   eight. The array will specify the eight chars building up the border
 ---   in a clockwise fashion starting with the top-left corner. As an
 ---   example, the double box style could be specified as:

runtime/lua/vim/_meta/options.lua

@@ -7778,6 +7778,19 @@ vim.o.winbl = vim.o.winblend
 vim.wo.winblend = vim.o.winblend
 vim.wo.winbl = vim.wo.winblend
 
+--- Defines the default border style of floating windows. The default value
+--- is empty, which is equivalent to "none". Valid values include:
+--- - "none": No border.
+--- - "single": A single line box.
+--- - "double": A double line box.
+--- - "rounded": Like "single", but with rounded corners ("╭" etc.).
+--- - "solid": Adds padding by a single whitespace cell.
+--- - "shadow": A drop shadow effect by blending with the background.
+---
+--- @type string
+vim.o.winborder = ""
+vim.go.winborder = vim.o.winborder
+
 --- Window height used for `CTRL-F` and `CTRL-B` when there is only one
 --- window and the value is smaller than 'lines' minus one.  The screen
 --- will scroll 'window' minus two lines, with a minimum of one.

src/nvim/api/win_config.c

@@ -160,14 +160,8 @@
 ///                    'fillchars' to a space char, and clearing the
 ///                    |hl-EndOfBuffer| region in 'winhighlight'.
 ///   - border: Style of (optional) window border. This can either be a string
-///     or an array. The string values are
+///     or an array. The string values are the same as those described in 'winborder'.
-///     - "none": No border (default).
+///     If it is an array, it should have a length of eight or any divisor of
-///     - "single": A single line box.
-///     - "double": A double line box.
-///     - "rounded": Like "single", but with rounded corners ("╭" etc.).
-///     - "solid": Adds padding by a single whitespace cell.
-///     - "shadow": A drop shadow effect by blending with the background.
-///     - If it is an array, it should have a length of eight or any divisor of
 ///     eight. The array will specify the eight chars building up the border
 ///     in a clockwise fashion starting with the top-left corner. As an
 ///     example, the double box style could be specified as:
@@ -937,11 +931,11 @@ static void parse_border_style(Object style, WinConfig *fconfig, Error *err)
     char chars[8][MAX_SCHAR_SIZE];
     bool shadow_color;
   } defaults[] = {
-    { "double", { "╔", "═", "╗", "║", "╝", "═", "╚", "║" }, false },
+    { opt_winborder_values[0], { "╔", "═", "╗", "║", "╝", "═", "╚", "║" }, false },
-    { "single", { "┌", "─", "┐", "│", "┘", "─", "└", "│" }, false },
+    { opt_winborder_values[1], { "┌", "─", "┐", "│", "┘", "─", "└", "│" }, false },
-    { "shadow", { "", "", " ", " ", " ", " ", " ", "" }, true },
+    { opt_winborder_values[2], { "", "", " ", " ", " ", " ", " ", "" }, true },
-    { "rounded", { "╭", "─", "╮", "│", "╯", "─", "╰", "│" }, false },
+    { opt_winborder_values[3], { "╭", "─", "╮", "│", "╯", "─", "╰", "│" }, false },
-    { "solid", { " ", " ", " ", " ", " ", " ", " ", " " }, false },
+    { opt_winborder_values[4], { " ", " ", " ", " ", " ", " ", " ", " " }, false },
     { NULL, { { NUL } }, false },
   };
 
@@ -1282,12 +1276,13 @@ static bool parse_win_config(win_T *wp, Dict(win_config) *config, WinConfig *fco
     }
   }
 
-  if (HAS_KEY_X(config, border)) {
+  if (HAS_KEY_X(config, border) || *p_winbd != NUL) {
     if (is_split) {
       api_set_error(err, kErrorTypeValidation, "non-float cannot have 'border'");
       goto fail;
     }
-    parse_border_style(config->border, fconfig, err);
+    Object style = config->border.type != kObjectTypeNil ? config->border : CSTR_AS_OBJ(p_winbd);
+    parse_border_style(style, fconfig, err);
     if (ERROR_SET(err)) {
       goto fail;
     }

src/nvim/option_vars.h

@@ -608,6 +608,7 @@ EXTERN OptInt p_wcm;            ///< 'wildcharm'
 EXTERN int p_wic;               ///< 'wildignorecase'
 EXTERN char *p_wim;             ///< 'wildmode'
 EXTERN int p_wmnu;              ///< 'wildmenu'
+EXTERN char *p_winbd;           ///< 'winborder'
 EXTERN OptInt p_wh;             ///< 'winheight'
 EXTERN OptInt p_wmh;            ///< 'winminheight'
 EXTERN OptInt p_wmw;            ///< 'winminwidth'

src/nvim/options.lua

@@ -10167,6 +10167,26 @@ return {
       short_desc = N_('Controls transparency level for floating windows'),
       type = 'number',
     },
+    {
+      defaults = { if_true = '' },
+      cb = 'did_set_winborder',
+      values = { 'double', 'single', 'shadow', 'rounded', 'solid', 'none' },
+      desc = [=[
+        Defines the default border style of floating windows. The default value
+        is empty, which is equivalent to "none". Valid values include:
+        - "none": No border.
+        - "single": A single line box.
+        - "double": A double line box.
+        - "rounded": Like "single", but with rounded corners ("╭" etc.).
+        - "solid": Adds padding by a single whitespace cell.
+        - "shadow": A drop shadow effect by blending with the background.
+      ]=],
+      full_name = 'winborder',
+      scope = { 'global' },
+      short_desc = N_('border of floating window'),
+      type = 'string',
+      varname = 'p_winbd',
+    },
     {
       abbreviation = 'wi',
       cb = 'did_set_window',

src/nvim/optionstr.c

@@ -2518,6 +2518,15 @@ const char *did_set_winhighlight(optset_T *args)
   return NULL;
 }
 
+/// The 'winborder' option is changed.
+const char *did_set_winborder(optset_T *args)
+{
+  if (check_opt_strings(p_winbd, opt_winborder_values, true) != OK) {
+    return e_invarg;
+  }
+  return NULL;
+}
+
 int expand_set_winhighlight(optexpand_T *args, int *numMatches, char ***matches)
 {
   return expand_set_opt_generic(args, get_highlight_name, numMatches, matches);

test/functional/ui/float_spec.lua

@@ -9514,6 +9514,116 @@ describe('float window', function()
         })
       end
     end)
+
+    it('#winborder option', function()
+      local buf = api.nvim_create_buf(false,false)
+      local config = {relative='editor', width=4, height=4, row=2, col=2}
+      command('set winborder=single')
+      api.nvim_open_win(buf, true, config)
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            [2:----------------------------------------]|*6
+            [3:----------------------------------------]|
+          ## grid 2
+                                                    |
+            {0:~                                       }|*5
+          ## grid 3
+                                                    |
+          ## grid 4
+            {5:┌────┐}|
+            {5:│}{1:^    }{5:│}|
+            {5:│}{2:~   }{5:│}|*3
+            {5:└────┘}|
+          ]], float_pos={
+          [4] = {1001, "NW", 1, 2, 2, true, 50};
+        }, win_viewport={
+          [2] = {win = 1000, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [4] = {win = 1001, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+        }, win_viewport_margins={
+          [2] = {
+            bottom = 0,
+            left = 0,
+            right = 0,
+            top = 0,
+            win = 1000
+          },
+          [4] = {
+            bottom = 1,
+            left = 1,
+            right = 1,
+            top = 1,
+            win = 1001
+          }
+        }
+        })
+      else
+        screen:expect({
+          grid = [[
+              {5:┌────┐}                                |
+            {0:~ }{5:│}{1:^    }{5:│}{0:                                }|
+            {0:~ }{5:│}{2:~   }{5:│}{0:                                }|*3
+            {0:~ }{5:└────┘}{0:                                }|
+                                                    |
+          ]]
+        })
+      end
+
+      command('fclose')
+      command('set winborder=double')
+      api.nvim_open_win(buf, true, config)
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            [2:----------------------------------------]|*6
+            [3:----------------------------------------]|
+          ## grid 2
+                                                    |
+            {0:~                                       }|*5
+          ## grid 3
+                                                    |
+          ## grid 5
+            {5:╔════╗}|
+            {5:║}{1:^    }{5:║}|
+            {5:║}{2:~   }{5:║}|*3
+            {5:╚════╝}|
+          ]], float_pos={
+          [5] = {1002, "NW", 1, 2, 2, true, 50};
+        }, win_viewport={
+          [2] = {win = 1000, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = 1002, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+        }, win_viewport_margins={
+          [2] = {
+            bottom = 0,
+            left = 0,
+            right = 0,
+            top = 0,
+            win = 1000
+          },
+          [5] = {
+            bottom = 1,
+            left = 1,
+            right = 1,
+            top = 1,
+            win = 1002
+          }
+        }
+        })
+      else
+        screen:expect({
+          grid = [[
+              {5:╔════╗}                                |
+            {0:~ }{5:║}{1:^    }{5:║}{0:                                }|
+            {0:~ }{5:║}{2:~   }{5:║}{0:                                }|*3
+            {0:~ }{5:╚════╝}{0:                                }|
+                                                    |
+          ]]
+        })
+      end
+      command('fclose!')
+    end)
   end
 
   describe('with ext_multigrid', function()

test/old/testdir/gen_opt_test.vim

@@ -75,6 +75,7 @@ let test_values = {
       \ 'shada': [['', '''50', '"30'], ['xxx']],
       \ 'termpastefilter': [['BS', 'HT', 'FF', 'ESC', 'DEL', 'C0', 'C1', 'C0,C1'],
       \		['xxx', 'C0,C1,xxx']],
+      \ 'winborder': [['rounded', 'none', 'single', 'solid'], ['xxx']],
       \ 'winhighlight': [['', 'a:b', 'a:', 'a:b,c:d'],
       \		['a', ':', ':b', 'a:b:c', 'a:/', '/:b', ',', 'a:b,,', 'a:b,c']],
       \