patch 8.1.1589: popup window does not indicate scroll position

Problem:    Popup window does not indicate scroll position.
Solution:   Add a scrollbar.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index b4bfcef..1e8f522 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -1407,12 +1407,42 @@
   call term_sendkeys(buf, ":call popup_notification('another important notification', {})\<CR>")
   call VerifyScreenDump(buf, 'Test_popupwin_notify_02', {})
 
-
   " clean up
   call StopVimInTerminal(buf)
   call delete('XtestNotifications')
 endfunc
 
+func Test_popup_scrollbar()
+  if !CanRunVimInTerminal()
+    throw 'Skipped: cannot make screendumps'
+  endif
+
+  let lines =<< trim END
+    call setline(1, range(1, 20))
+    let winid = popup_create(['one', 'two', 'three', 'four', 'five',
+	  \ 'six', 'seven', 'eight', 'nine'], {
+	  \ 'minwidth': 8,
+	  \ 'maxheight': 4,
+	  \ })
+  END
+  call writefile(lines, 'XtestPopupScroll')
+  let buf = RunVimInTerminal('-S XtestPopupScroll', {'rows': 10})
+  call VerifyScreenDump(buf, 'Test_popupwin_scroll_1', {})
+
+  call term_sendkeys(buf, ":call popup_setoptions(winid, {'firstline': 2})\<CR>")
+  call VerifyScreenDump(buf, 'Test_popupwin_scroll_2', {})
+
+  call term_sendkeys(buf, ":call popup_setoptions(winid, {'firstline': 6})\<CR>")
+  call VerifyScreenDump(buf, 'Test_popupwin_scroll_3', {})
+
+  call term_sendkeys(buf, ":call popup_setoptions(winid, {'firstline': 9})\<CR>")
+  call VerifyScreenDump(buf, 'Test_popupwin_scroll_4', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('XtestPopupScroll')
+endfunc
+
 func Test_popup_settext()
   if !CanRunVimInTerminal()
     throw 'Skipped: cannot make screendumps'