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

test/functional/legacy/display_spec.lua

@@ -0,0 +1,31 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local Screen = require('test.functional.ui.screen')
+local clear = helpers.clear
+local wait = helpers.wait
+local feed = helpers.feed
+local feed_command = helpers.feed_command
+
+describe('display', function()
+  local screen
+
+  it('scroll when modified at topline', function()
+    clear()
+    screen = Screen.new(20, 4)
+    screen:attach()
+    screen:set_default_attr_ids({
+      [1] = {bold = true},
+    })
+
+    feed_command([[call setline(1, repeat('a', 21))]])
+    wait()
+    feed('O')
+    screen:expect([[
+      ^                    |
+      aaaaaaaaaaaaaaaaaaaa|
+      a                   |
+      {1:-- INSERT --}        |
+    ]])
+  end)
+end)
+