patch 8.2.2508: cannot change the character displayed in non existing lines

Problem:    Cannot change the character displayed in non existing lines.
Solution:   Add the "eob" item to 'fillchars'. (closes #7832, closes #3820)
diff --git a/src/testdir/test_display.vim b/src/testdir/test_display.vim
index bfcb485..5bf9e5e 100644
--- a/src/testdir/test_display.vim
+++ b/src/testdir/test_display.vim
@@ -257,4 +257,26 @@
   call StopVimInTerminal(buf)
 endfunc
 
+" Test for 'eob' (EndOfBuffer) item in 'fillchars'
+func Test_eob_fillchars()
+  " default value
+  call assert_match('eob:\~', &fillchars)
+  " invalid values
+  call assert_fails(':set fillchars=eob:', 'E474:')
+  call assert_fails(':set fillchars=eob:xy', 'E474:')
+  call assert_fails(':set fillchars=eob:\255', 'E474:')
+  call assert_fails(':set fillchars=eob:<ff>', 'E474:')
+  " default is ~
+  new
+  call assert_equal('~', Screenline(2))
+  set fillchars=eob:+
+  redraw!
+  call assert_equal('+', Screenline(2))
+  set fillchars=eob:\ 
+  redraw!
+  call assert_equal(' ', nr2char(screenchar(2, 1)))
+  set fillchars&
+  close
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab