Popup preview window and insert completion popup windows not refreshed properly

[Yegappan Lakshmanan]

Hi,

When I set the 'previewpopup' option and open the preview window and
then display an overlapping insert-mode completion popup window, then
the popup window contents are not correctly refreshed (when the
insert completion popup window contents are scrolled).

I see the problem with the following commands and steps:

enew!
call setline(1, ['one', 'two', 'three'])
set previewpopup=height:10,width:50
pedit $MYVIMRC

Now open the insert completion popup menu after line 3 and press
Ctrl-N and Ctrl-P several times to scroll the popup menu. The popup
contents are not properly refreshed. After canceling the insert
completion popup menu, you will notice that the preview popup is
also not properly refreshed.

- Yegappan