feat: add support for global statusline

Ref: #9342

Adds the option to have a single global statusline for the current window at the bottom of the screen instead of a statusline at the bottom of every window. Enabled by setting `laststatus = 3`.

Due to the fact that statuslines at the bottom of windows are removed when global statusline is enabled, horizontal separators are used instead to separate horizontal splits. The horizontal separator character is configurable through the`horiz` item in `'fillchars'`. Separator connector characters are also used to connect the horizontal and vertical separators together, which are also configurable through the `horizup`, `horizdown`, `vertleft`, `vertright` and `verthoriz` items in `fillchars`.

The window separators are highlighted using the `WinSeparator` highlight group, which supersedes `VertSplit` and is linked to `VertSplit` by default in order to maintain backwards compatibility.

6 files changed, 36 insertions, 8 deletions

diff --git a/runtime/doc/arabic.txt b/runtime/doc/arabic.txt
index 5d3bf7a761..0df861111c 100644
--- a/runtime/doc/arabic.txt
+++ b/runtime/doc/arabic.txt
@@ -175,7 +175,7 @@ o  Enable Arabic settings [short-cut]
 	 vertical separator like "l" or "𝖨" may be used.  It may also be
 	 hidden by changing its color to the foreground color: >
 		:set fillchars=vert:l
-		:hi VertSplit ctermbg=White
+		:hi WinSeparator ctermbg=White
 <	Note that this is a workaround, not a proper solution.
 
    If, on the other hand, you'd like to be verbose and explicit and
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 6e2bc228d0..66048c2e5f 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -2441,7 +2441,14 @@ A jump table for the options with a short description can be found at |Q_op|.
 	  item		default		Used for ~
 	  stl:c		' ' or '^'	statusline of the current window
 	  stlnc:c	' ' or '='	statusline of the non-current windows
+	  horiz:c	'─' or '-'	horizontal separators |:split|
+	  horizup:c	'┴' or '-'	upwards facing horizontal separator
+	  horizdown:c	'┬' or '-'	downwards facing horizontal separator
 	  vert:c	'│' or '|'	vertical separators |:vsplit|
+	  vertleft:c	'┤' or '|'	left facing vertical separator
+	  vertright:c	'├' or '|'	right facing vertical separator
+	  verthoriz:c	'┼' or '+'	overlapping vertical and horizontal
+					separator
 	  fold:c	'·' or '-'	filling 'foldtext'
 	  foldopen:c	'-'		mark the beginning of a fold
 	  foldclose:c	'+'		show a closed fold
@@ -2454,8 +2461,13 @@ A jump table for the options with a short description can be found at |Q_op|.
 	"stlnc" the space will be used when there is highlighting, '^' or '='
 	otherwise.
 
-	If 'ambiwidth' is "double" then "vert", "foldsep" and "fold" default to
-	single-byte alternatives.
+	Note that "horiz", "horizup", "horizdown", "vertleft", "vertright" and
+	"verthoriz" are only used when 'laststatus' is 3, since only vertical
+	window separators are used otherwise.
+
+	If 'ambiwidth' is "double" then "horiz", "horizup", "horizdown",
+	"vert", "vertleft", "vertright", "verthoriz", "foldsep" and "fold"
+	default to single-byte alternatives.
 
 	Example: >
 	    :set fillchars=stl:^,stlnc:=,vert:│,fold:·,diff:-
@@ -2469,7 +2481,13 @@ A jump table for the options with a short description can be found at |Q_op|.
 	  item		highlight group ~
 	  stl:c		StatusLine		|hl-StatusLine|
 	  stlnc:c	StatusLineNC		|hl-StatusLineNC|
-	  vert:c	VertSplit		|hl-VertSplit|
+	  horiz:c	WinSeparator		|hl-WinSeparator|
+	  horizup:c	WinSeparator		|hl-WinSeparator|
+	  horizdown:c	WinSeparator		|hl-WinSeparator|
+	  vert:c	WinSeparator		|hl-WinSeparator|
+	  vertleft:c	WinSeparator		|hl-WinSeparator|
+	  vertright:c	WinSeparator		|hl-WinSeparator|
+	  verthoriz:c	WinSeparator		|hl-WinSeparator|
 	  fold:c	Folded			|hl-Folded|
 	  diff:c	DiffDelete		|hl-DiffDelete|
 	  eob:c		EndOfBuffer		|hl-EndOfBuffer|
@@ -3652,6 +3670,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 		0: never
 		1: only if there are at least two windows
 		2: always
+		3: have a global statusline at the bottom instead of one for
+		   each window
 	The screen looks nicer with a status line if you have several
 	windows, but it takes another screen line. |status-line|
 
@@ -5929,7 +5949,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	empty to avoid further errors.  Otherwise screen updating would loop.
 
 	Note that the only effect of 'ruler' when this option is set (and
-	'laststatus' is 2) is controlling the output of |CTRL-G|.
+	'laststatus' is 2 or 3) is controlling the output of |CTRL-G|.
 
 	field	    meaning ~
 	-	    Left justify the item.  The default is right justified
diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index 319a715e40..5446bd3a88 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -5105,8 +5105,8 @@ TermCursor	cursor in a focused terminal
 TermCursorNC	cursor in an unfocused terminal
 							*hl-ErrorMsg*
 ErrorMsg	error messages on the command line
-							*hl-VertSplit*
-VertSplit	the column separating vertically split windows
+							*hl-WinSeparator*
+WinSeparator	separators between window splits
 							*hl-Folded*
 Folded		line used for closed folds
 							*hl-FoldColumn*
diff --git a/runtime/doc/usr_08.txt b/runtime/doc/usr_08.txt
index 8ccaa73006..1d20913a14 100644
--- a/runtime/doc/usr_08.txt
+++ b/runtime/doc/usr_08.txt
@@ -482,6 +482,8 @@ statusline:
 	0	never
 	1	only when there are split windows (the default)
 	2	always
+	3	have a global statusline at the bottom instead of one for each
+		window
 
 Many commands that edit another file have a variant that splits the window.
 For Command-line commands this is done by prepending an "s".  For example:
diff --git a/runtime/doc/vim_diff.txt b/runtime/doc/vim_diff.txt
index 7892b82137..69b0e0081a 100644
--- a/runtime/doc/vim_diff.txt
+++ b/runtime/doc/vim_diff.txt
@@ -211,6 +211,7 @@ Highlight groups:
   |hl-Substitute|
   |hl-TermCursor|
   |hl-TermCursorNC|
+  |hl-WinSeparator| highlights window separators
   |hl-Whitespace| highlights 'listchars' whitespace
 
 Input/Mappings:
@@ -227,9 +228,11 @@ Options:
   'cpoptions'   flags: |cpo-_|
   'display'     flags: "msgsep" minimizes scrolling when showing messages
   'guicursor'   works in the terminal
-  'fillchars'   flags: "msgsep" (see 'display')
+  'fillchars'   flags: "msgsep" (see 'display'), "horiz", "horizup",
+                "horizdown", "vertleft", "vertright", "verthoriz"
   'foldcolumn'  supports up to 9 dynamic/fixed columns
   'inccommand'  shows interactive results for |:substitute|-like commands
+  'laststatus'  global statusline support
   'pumblend'    pseudo-transparent popupmenu
   'scrollback'
   'signcolumn'  supports up to 9 dynamic/fixed columns
@@ -348,6 +351,7 @@ Highlight groups:
   |hl-ColorColumn|, |hl-CursorColumn| are lower priority than most other
   groups
   |hl-CursorLine| is low-priority unless foreground color is set
+  *hl-VertSplit* superseded by |hl-WinSeparator|
 
 Macro/|recording| behavior
   Replay of a macro recorded during :lmap produces the same actions as when it
diff --git a/runtime/doc/windows.txt b/runtime/doc/windows.txt
index 5b91321c40..f8a8a557ef 100644
--- a/runtime/doc/windows.txt
+++ b/runtime/doc/windows.txt
@@ -104,6 +104,8 @@ when the last window also has a status line:
 	'laststatus' = 0	never a status line
 	'laststatus' = 1	status line if there is more than one window
 	'laststatus' = 2	always a status line
+	'laststatus' = 3	have a global statusline at the bottom instead
+				of one for each window
 
 You can change the contents of the status line with the 'statusline' option.
 This option can be local to the window, so that you can have a different