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 #6739)
diff --git a/src/testdir/dumps/Test_display_scroll_at_topline.dump b/src/testdir/dumps/Test_display_scroll_at_topline.dump
new file mode 100644
index 0000000..9c843c0
--- /dev/null
+++ b/src/testdir/dumps/Test_display_scroll_at_topline.dump
@@ -0,0 +1,4 @@
+> +0&#ffffff0@19
+|a@19
+@1| @18
+|~+0#4040ff13&| @18
diff --git a/src/testdir/test_display.vim b/src/testdir/test_display.vim
index 97b0a9e..49d0d8c 100644
--- a/src/testdir/test_display.vim
+++ b/src/testdir/test_display.vim
@@ -245,4 +245,16 @@
   call delete(filename)
 endfunc
 
+func Test_display_scroll_at_topline()
+  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
+
 " vim: shiftwidth=2 sts=2 expandtab