patch 8.1.1770: cannot get the window ID of the popup preview window
Problem: Cannot get the window ID of the popup preview window.
Solution: Add popup_getpreview().
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 79c5409..bd05691 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2554,6 +2554,7 @@
popup_filter_yesno({id}, {key}) Number filter for a dialog popup window
popup_getoptions({id}) Dict get options of popup window {id}
popup_getpos({id}) Dict get position of popup window {id}
+popup_getpreview() Number get window ID of preview popup window
popup_hide({id}) none hide popup menu {id}
popup_menu({what}, {options}) Number create a popup window used as a menu
popup_move({id}, {options}) none set position of popup window {id}