patch 9.1.0034: Window scrolls unexpectedly when 'scrollbind' is set

Problem:  Window may unexpectedly scroll when 'scrollbind' is set
          and setting a buffer-local option using setbufvar()
          (Boris Staletic)
Solution: Save and restore the windows topline before opening the
          popup window.

fixes: #13863
closes: #13869

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/evalvars.c b/src/evalvars.c
index 8e42c5a..cbcf2a7 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -4924,6 +4924,10 @@
     if (*varname == '&')
     {
 	aco_save_T	aco;
+	// safe the current window position, it could
+	// change because of 'scrollbind' window-local
+	// options
+	linenr_T	old_topline = curwin->w_topline;
 
 	// Set curbuf to be our buf, temporarily.
 	aucmd_prepbuf(&aco, buf);
@@ -4935,6 +4939,7 @@
 	    // reset notion of buffer
 	    aucmd_restbuf(&aco);
 	}
+	curwin->w_topline = old_topline;
     }
     else
     {