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/dumps/Test_popupwin_firstline.dump b/src/testdir/dumps/Test_popupwin_firstline.dump
index 276f4e5..7efcf3e 100644
--- a/src/testdir/dumps/Test_popupwin_firstline.dump
+++ b/src/testdir/dumps/Test_popupwin_firstline.dump
@@ -1,10 +1,10 @@
 >1+0&#ffffff0| @73
 |2| @73
 |3| @73
-|4| @33|3+0#0000001#ffd7ff255@4| +0#0000000#ffffff0@34
-|5| @33|4+0#0000001#ffd7ff255@1| @2| +0#0000000#ffffff0@34
-|6| @33|5+0#0000001#ffd7ff255| @3| +0#0000000#ffffff0@34
-|7| @33|6+0#0000001#ffd7ff255@4| +0#0000000#ffffff0@34
+|4| @33|3+0#0000001#ffd7ff255@4| +0#0000000#a8a8a8255| +0&#ffffff0@33
+|5| @33|4+0#0000001#ffd7ff255@1| @2| +0#0000000#0000001| +0&#ffffff0@33
+|6| @33|5+0#0000001#ffd7ff255| @3| +0#0000000#a8a8a8255| +0&#ffffff0@33
+|7| @33|6+0#0000001#ffd7ff255@4| +0#0000000#a8a8a8255| +0&#ffffff0@33
 |8| @73
 |9| @73
 @57|1|,|1| @10|T|o|p|