Add ‘eob’ option to fillchars

This option allows configuring what character is shown on the empty
lines at the end of a buffer, previously hardcoded to ‘~’

runtime/doc/options.txt

@@ -2393,6 +2393,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	  fold:c	'·' or '-'	filling 'foldtext'
 	  diff:c	'-'		deleted lines of the 'diff' option
 	  msgsep:c	' '		message separator 'display'
+	  eob:c		'~'		empty lines at the end of a buffer
 
 	Any one that is omitted will fall back to the default.  For "stl" and
 	"stlnc" the space will be used when there is highlighting, '^' or '='
@@ -2415,6 +2416,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	  vert:c	VertSplit		|hl-VertSplit|
 	  fold:c	Folded			|hl-Folded|
 	  diff:c	DiffDelete		|hl-DiffDelete|
+	  eob:c		EndOfBuffer		|hl-EndOfBuffer|
 
 		*'fixendofline'* *'fixeol'* *'nofixendofline'* *'nofixeol'*
 'fixendofline' 'fixeol'	boolean	(default on)

src/nvim/globals.h

@@ -908,6 +908,7 @@ EXTERN int fill_vert INIT(= 9474);  // │
 EXTERN int fill_fold INIT(= 183);   // ·
 EXTERN int fill_diff INIT(= '-');
 EXTERN int fill_msgsep INIT(= ' ');
+EXTERN int fill_eob INIT(= '~');
 
 /* Whether 'keymodel' contains "stopsel" and "startsel". */
 EXTERN int km_stopsel INIT(= FALSE);

src/nvim/option.c

@@ -3386,6 +3386,7 @@ static char_u *set_chars_option(char_u **varp)
     { &fill_fold,    "fold" , 183  },  // ·
     { &fill_diff,    "diff" , '-'  },
     { &fill_msgsep,  "msgsep", ' ' },
+    { &fill_eob,     "eob",   '~' },
   };
   static struct charstab lcstab[] = {
     { &lcs_eol,      "eol",      NUL },

src/nvim/screen.c

@@ -1532,9 +1532,9 @@ static void win_update(win_T *wp)
     } else if (dollar_vcol == -1)
       wp->w_botline = lnum;
 
-    /* make sure the rest of the screen is blank */
-    /* put '~'s on rows that aren't part of the file. */
-    win_draw_end(wp, '~', ' ', row, wp->w_height, HLF_EOB);
+    // make sure the rest of the screen is blank
+    // write the 'fill_eob' character to rows that aren't part of the file.
+    win_draw_end(wp, fill_eob, ' ', row, wp->w_height, HLF_EOB);
   }
 
   /* Reset the type of redrawing required, the window has been updated. */

test/functional/options/fillchars_spec.lua

@@ -0,0 +1,45 @@
+local helpers = require('test.functional.helpers')(after_each)
+local Screen = require('test.functional.ui.screen')
+local clear, execute = helpers.clear, helpers.execute
+
+describe("'fillchars'", function()
+  local screen
+
+  before_each(function()
+    clear()
+    screen = Screen.new(25, 5)
+    screen:attach()
+  end)
+
+  after_each(function()
+    screen:detach()
+  end)
+
+  describe('"eob" flag', function()
+    it('renders empty lines at the end of the buffer with eob', function()
+      screen:expect([[
+        ^                         |
+        ~                        |
+        ~                        |
+        ~                        |
+                                 |
+      ]])
+      execute('set fillchars+=eob:\\ ')
+      screen:expect([[
+        ^                         |
+                                 |
+                                 |
+                                 |
+        :set fillchars+=eob:\    |
+      ]])
+      execute('set fillchars+=eob:ñ')
+      screen:expect([[
+        ^                         |
+        ñ                        |
+        ñ                        |
+        ñ                        |
+        :set fillchars+=eob:ñ    |
+      ]])
+    end)
+  end)
+end)