patch 8.1.1804: no test for display updating without a scroll region

Problem:    No test for display updating without a scroll region.
Solution:   Add a test.
diff --git a/src/testdir/test_display.vim b/src/testdir/test_display.vim
index 81ffb2c..dfa641b 100644
--- a/src/testdir/test_display.vim
+++ b/src/testdir/test_display.vim
@@ -4,11 +4,12 @@
 endif
 
 source view_util.vim
+source check.vim
+source screendump.vim
 
 func Test_display_foldcolumn()
-  if !has("folding")
-    return
-  endif
+  CheckFeature folding
+
   new
   vnew
   vert resize 25
@@ -24,10 +25,10 @@
 
   call cursor(2, 1)
   norm! zt
-  let lines=ScreenLines([1,2], winwidth(0))
+  let lines = ScreenLines([1,2], winwidth(0))
   call assert_equal(expect, lines)
   set fdc=2
-  let lines=ScreenLines([1,2], winwidth(0))
+  let lines = ScreenLines([1,2], winwidth(0))
   let expect = [
         \ "  e more noise blah blah<",
         \ "  82> more stuff here    "
@@ -39,9 +40,8 @@
 endfunc
 
 func Test_display_foldtext_mbyte()
-  if !has("folding")
-    return
-  endif
+  CheckFeature folding
+
   call NewWindow(10, 40)
   call append(0, range(1,20))
   exe "set foldmethod=manual foldtext=foldtext() fillchars=fold:\u2500,vert:\u2502 fdc=2"
@@ -67,3 +67,28 @@
   set foldtext& fillchars& foldmethod& fdc&
   bw!
 endfunc
+
+" check that win_ins_lines() and win_del_lines() work when t_cs is empty.
+func Test_scroll_without_region()
+  CheckScreendump
+
+  let lines =<< trim END
+    call setline(1, range(1, 20))
+    set t_cs=
+    set laststatus=2
+  END
+  call writefile(lines, 'Xtestscroll')
+  let buf = RunVimInTerminal('-S Xtestscroll', #{rows: 10})
+
+  call VerifyScreenDump(buf, 'Test_scroll_no_region_1', {})
+
+  call term_sendkeys(buf, ":3delete\<cr>")
+  call VerifyScreenDump(buf, 'Test_scroll_no_region_2', {})
+
+  call term_sendkeys(buf, ":4put\<cr>")
+  call VerifyScreenDump(buf, 'Test_scroll_no_region_3', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('Xtestscroll')
+endfunc