patch 8.1.1525: cannot move a popup window with the mouse
Problem: Cannot move a popup window with the mouse.
Solution: Add the "drag" property and make it possible to drag a popup
window by its border.
diff --git a/src/proto/popupwin.pro b/src/proto/popupwin.pro
index 03d3729..eeabafa 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -1,4 +1,7 @@
/* popupwin.c */
+int popup_on_border(win_T *wp, int row, int col);
+void popup_start_drag(win_T *wp);
+void popup_drag(win_T *wp);
int popup_height(win_T *wp);
int popup_width(win_T *wp);
void popup_adjust_position(win_T *wp);