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>

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:
 ---     ```
 ---     [ "╔", "═" ,"╗", "║", "╝", "═", "╚", "║" ].
 ---     ```

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.

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,