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>

3 files changed, 25 insertions, 20 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