patch 8.1.1421: drawing "~" line in popup window

Problem:    Drawing "~" line in popup window.
Solution:   Just draw text in the last line of the popup window.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index a2d2e80..4511cd1 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -41,11 +41,16 @@
   call term_sendkeys(buf, ":quit!\<CR>")
   call VerifyScreenDump(buf, 'Test_popupwin_04', {})
 
-  " resize popup
+  " resize popup, show empty line at bottom
   call term_sendkeys(buf, ":call popup_move(popupwin, {'minwidth': 15, 'maxwidth': 25, 'minheight': 3, 'maxheight': 5})\<CR>")
   call term_sendkeys(buf, ":redraw\<CR>")
   call VerifyScreenDump(buf, 'Test_popupwin_05', {})
 
+  " show not fitting line at bottom
+  call term_sendkeys(buf, ":call setbufline(winbufnr(popupwin), 3, 'this line will not fit here')\<CR>")
+  call term_sendkeys(buf, ":redraw\<CR>")
+  call VerifyScreenDump(buf, 'Test_popupwin_06', {})
+
   " clean up
   call StopVimInTerminal(buf)
   call delete('XtestPopup')