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>

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;
     }

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'

src/nvim/options.lua

@@ -10188,6 +10188,26 @@ local options = {
       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

@@ -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);