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)

src/drawscreen.c

@@ -1662,7 +1662,7 @@ 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.

src/testdir/dumps/Test_display_scroll_at_topline.dump

@@ -0,0 +1,4 @@
+> +0&#ffffff0@19
+|a@19
+@1| @18
+|~+0#4040ff13&| @18

src/testdir/test_display.vim

@@ -245,4 +245,16 @@ func Test_visual_block_scroll()
   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

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1488,
 /**/
     1487,
 /**/