feat(float): add winborder option (#31074)

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.

Co-authored-by: Gregory Anders <greg@gpanders.com>

12 files changed, 340 insertions, 62 deletions

diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index dae4df00af..eba4bc9562 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -3493,28 +3493,20 @@ 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
-                    • "none": No border (default).
-                    • "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: >
-                      [ "╔", "═" ,"╗", "║", "╝", "═", "╚", "║" ].
+                    be a string or an array. The string values are the same as
+                    those described in 'winborder'. 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
-                      repeated. Thus an ASCII border could be specified as >
-                      [ "/", "-", \"\\\\\", "|" ],
+                    If the number of chars are less than eight, they will be
+                    repeated. Thus an ASCII border could be specified as >
+                     [ "/", "-", \"\\\\\", "|" ],
 <
-                      or all chars the same as >
-                      [ "x" ].
+                    or all chars the same as >
+                     [ "x" ].
 <
                     An empty string can be used to turn off a specific border,
                     for instance, >
diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index 8c7e400042..a5305e5903 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -348,6 +348,7 @@ OPTIONS
 • 'messagesopt' configures |:messages| and |hit-enter| prompt.
 • 'tabclose' controls which tab page to focus when closing a tab page.
 • 'eventignorewin' to persistently ignore events in a window.
+• 'winborder' sets the default border for |floating-windows|
 
 PERFORMANCE
 
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 01d5efee4a..ba1d353f40 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -7150,6 +7150,18 @@ 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
diff --git a/runtime/lua/vim/_meta/api.lua b/runtime/lua/vim/_meta/api.lua
index 2c1fb260b2..e7ad91132d 100644
--- a/runtime/lua/vim/_meta/api.lua
+++ b/runtime/lua/vim/_meta/api.lua
@@ -1797,17 +1797,11 @@ 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
----   - "none": No border (default).
----   - "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:
+---   or an array. The string values are the same as those described in 'winborder'.
+---   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:
 ---     ```
 ---     [ "╔", "═" ,"╗", "║", "╝", "═", "╚", "║" ].
 ---     ```
diff --git a/runtime/lua/vim/_meta/options.lua b/runtime/lua/vim/_meta/options.lua
index 5e138c1a4a..46366d585b 100644
--- a/runtime/lua/vim/_meta/options.lua
+++ b/runtime/lua/vim/_meta/options.lua
@@ -7832,6 +7832,19 @@ 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 ''|'double'|'single'|'shadow'|'rounded'|'solid'|'none'
+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.
diff --git a/runtime/lua/vim/lsp/util.lua b/runtime/lua/vim/lsp/util.lua
index 056cb0c73c..d8a6a70ccd 100644
--- a/runtime/lua/vim/lsp/util.lua
+++ b/runtime/lua/vim/lsp/util.lua
@@ -6,17 +6,6 @@ local uv = vim.uv
 
 local M = {}
 
-local default_border = {
-  { '', 'NormalFloat' },
-  { '', 'NormalFloat' },
-  { '', 'NormalFloat' },
-  { ' ', 'NormalFloat' },
-  { '', 'NormalFloat' },
-  { '', 'NormalFloat' },
-  { '', 'NormalFloat' },
-  { ' ', 'NormalFloat' },
-}
-
 --- @param border string|(string|[string,string])[]
 local function border_error(border)
   error(
@@ -43,7 +32,11 @@ local border_size = {
 --- @return integer height
 --- @return integer width
 local function get_border_size(opts)
-  local border = opts and opts.border or default_border
+  local border = opts and opts.border or vim.o.winborder
+
+  if border == '' then
+    border = 'none'
+  end
 
   if type(border) == 'string' then
     if not border_size[border] then
@@ -884,7 +877,7 @@ function M.make_floating_popup_options(width, height, opts)
       or 'cursor',
     style = 'minimal',
     width = width,
-    border = opts.border or default_border,
+    border = opts.border,
     zindex = opts.zindex or (api.nvim_win_get_config(0).zindex or 49) + 1,
     title = title,
     title_pos = title_pos,
diff --git a/src/nvim/api/win_config.c b/src/nvim/api/win_config.c
index 12bbd79dbd..31de5ca1eb 100644
--- a/src/nvim/api/win_config.c
+++ b/src/nvim/api/win_config.c
@@ -159,17 +159,11 @@
 ///                    '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
-///     - "none": No border (default).
-///     - "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:
+///     or an array. The string values are the same as those described in 'winborder'.
+///     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:
 ///       ```
 ///       [ "╔", "═" ,"╗", "║", "╝", "═", "╚", "║" ].
 ///       ```
@@ -944,11 +938,11 @@ static void parse_border_style(Object style, WinConfig *fconfig, Error *err)
     char chars[8][MAX_SCHAR_SIZE];
     bool shadow_color;
   } defaults[] = {
-    { "double", { "╔", "═", "╗", "║", "╝", "═", "╚", "║" }, false },
-    { "single", { "┌", "─", "┐", "│", "┘", "─", "└", "│" }, false },
-    { "shadow", { "", "", " ", " ", " ", " ", " ", "" }, true },
-    { "rounded", { "╭", "─", "╮", "│", "╯", "─", "╰", "│" }, false },
-    { "solid", { " ", " ", " ", " ", " ", " ", " ", " " }, false },
+    { opt_winborder_values[1], { "╔", "═", "╗", "║", "╝", "═", "╚", "║" }, false },
+    { opt_winborder_values[2], { "┌", "─", "┐", "│", "┘", "─", "└", "│" }, false },
+    { opt_winborder_values[3], { "", "", " ", " ", " ", " ", " ", "" }, true },
+    { opt_winborder_values[4], { "╭", "─", "╮", "│", "╯", "─", "╰", "│" }, false },
+    { opt_winborder_values[5], { " ", " ", " ", " ", " ", " ", " ", " " }, false },
     { NULL, { { NUL } }, false },
   };
 
@@ -1279,12 +1273,13 @@ 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;
     }
diff --git a/src/nvim/option_vars.h b/src/nvim/option_vars.h
index 340a12a32f..a484eb8447 100644
--- a/src/nvim/option_vars.h
+++ b/src/nvim/option_vars.h
@@ -566,6 +566,7 @@ 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'
diff --git a/src/nvim/options.lua b/src/nvim/options.lua
index d150303033..3cf60513c4 100644
--- a/src/nvim/options.lua
+++ b/src/nvim/options.lua
@@ -10189,6 +10189,26 @@ local options = {
       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',
       defaults = {
diff --git a/src/nvim/optionstr.c b/src/nvim/optionstr.c
index 3a6b4c9936..c3cdd6e804 100644
--- a/src/nvim/optionstr.c
+++ b/src/nvim/optionstr.c
@@ -2005,6 +2005,15 @@ const char *did_set_winhighlight(optset_T *args)
   return NULL;
 }
 
+/// The 'winborder' option is changed.
+const char *did_set_winborder(optset_T *args)
+{
+  if (opt_strings_flags(p_winbd, opt_winborder_values, NULL, 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);
diff --git a/test/functional/ui/float_spec.lua b/test/functional/ui/float_spec.lua
index 6610d25afb..fa4937cec3 100644
--- a/test/functional/ui/float_spec.lua
+++ b/test/functional/ui/float_spec.lua
@@ -9909,6 +9909,253 @@ describe('float window', function()
       ]])
       eq({"%f", ""}, res)
     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!')
+
+      command('set winborder=none')
+      api.nvim_buf_set_lines(buf, 0, -1, false, {'none border'})
+      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 6
+            {1:^none}|
+            {1: bor}|
+            {1:der }|
+            {2:~   }|
+          ]],
+          win_pos = {
+          [2] = {
+            height = 6,
+            startcol = 0,
+            startrow = 0,
+            width = 40,
+            win = 1000
+          }
+        },
+          float_pos = {
+          [6] = {1003, "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};
+          [6] = {win = 1003, 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
+          },
+          [6] = {
+            bottom = 0,
+            left = 0,
+            right = 0,
+            top = 0,
+            win = 1003
+          }
+        },
+        })
+      else
+        screen:expect([[
+                                                  |
+          {0:~                                       }|
+          {0:~ }{1:^none}{0:                                  }|
+          {0:~ }{1: bor}{0:                                  }|
+          {0:~ }{1:der }{0:                                  }|
+          {0:~ }{2:~   }{0:                                  }|
+                                                  |
+        ]])
+      end
+      command('fclose!')
+
+      -- respect config.border
+      command('set winborder=rounded')
+      config.border = 'single'
+      api.nvim_open_win(buf, false, config)
+      if multigrid then
+        screen:expect({
+          grid = [[
+          ## grid 1
+            [2:----------------------------------------]|*6
+            [3:----------------------------------------]|
+          ## grid 2
+            ^                                        |
+            {0:~                                       }|*5
+          ## grid 3
+                                                    |
+          ## grid 7
+            {5:┌────┐}|
+            {5:│}{1:none}{5:│}|
+            {5:│}{1: bor}{5:│}|
+            {5:│}{1:der }{5:│}|
+            {5:│}{2:~   }{5:│}|
+            {5:└────┘}|
+          ]],
+          win_pos = {
+          [2] = {
+            height = 6,
+            startcol = 0,
+            startrow = 0,
+            width = 40,
+            win = 1000
+          }
+        },
+          float_pos = {
+          [7] = {1004, "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};
+          [7] = {win = 1004, 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
+          },
+          [7] = {
+            bottom = 1,
+            left = 1,
+            right = 1,
+            top = 1,
+            win = 1004
+          }
+        },
+        })
+      else
+        screen:expect([[
+          ^  {5:┌────┐}                                |
+          {0:~ }{5:│}{1:none}{5:│}{0:                                }|
+          {0:~ }{5:│}{1: bor}{5:│}{0:                                }|
+          {0:~ }{5:│}{1:der }{5:│}{0:                                }|
+          {0:~ }{5:│}{2:~   }{5:│}{0:                                }|
+          {0:~ }{5:└────┘}{0:                                }|
+                                                  |
+        ]])
+      end
+
+      -- it is currently not supported.
+      eq('Vim(set):E474: Invalid argument: winborder=custom', pcall_err(command, 'set winborder=custom'))
+    end)
   end
 
   describe('with ext_multigrid', function()
diff --git a/test/old/testdir/gen_opt_test.vim b/test/old/testdir/gen_opt_test.vim
index d1d27cf578..84a352a158 100644
--- a/test/old/testdir/gen_opt_test.vim
+++ b/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']],
       \