vim-patch:8.2.1488: text does not scroll when inserting above first line

Problem:    Text does not scroll when inserting above first line.
Solution:   Adjust off-by-one error. (Ken Takata, closes vim/vim#6739)
9dc1917f42

src/nvim/screen.c

@@ -902,7 +902,7 @@ static void win_update(win_T *wp)
     if (mod_top != 0
         && wp->w_topline == mod_top
         && (!wp->w_lines[0].wl_valid
-            || wp->w_topline == wp->w_lines[0].wl_lnum)) {
+            || wp->w_topline <= wp->w_lines[0].wl_lnum)) {
       // w_topline is the first changed line and window is not scrolled,
       // the scrolling from changed lines will be done further down.
     } else if (wp->w_lines[0].wl_valid

src/nvim/testdir/test_display.vim

@@ -208,3 +208,16 @@ func Test_visual_block_scroll()
   call StopVimInTerminal(buf)
   call delete(filename)
 endfunc
+
+func Test_display_scroll_at_topline()
+  " See test/functional/legacy/display_spec.lua
+  CheckScreendump
+
+  let buf = RunVimInTerminal('', #{cols: 20})
+  call term_sendkeys(buf, ":call setline(1, repeat('a', 21))\<CR>")
+  call term_wait(buf)
+  call term_sendkeys(buf, "O\<Esc>")
+  call VerifyScreenDump(buf, 'Test_display_scroll_at_topline', #{rows: 4})
+
+  call StopVimInTerminal(buf)
+endfunc