patch 9.0.1443: ending Insert mode when accessing a hidden prompt buffer

Problem:    Ending Insert mode when accessing a hidden prompt buffer.
Solution:   Don't stop Insert mode when it was active before. (closes #12237)
diff --git a/src/autocmd.c b/src/autocmd.c
index e5f5125..6460fcd 100644
--- a/src/autocmd.c
+++ b/src/autocmd.c
@@ -1566,6 +1566,8 @@
     aco->save_curwin_id = curwin->w_id;
     aco->save_curbuf = curbuf;
     aco->save_prevwin_id = prevwin == NULL ? 0 : prevwin->w_id;
+    aco->save_State = State;
+
     if (win != NULL)
     {
 	// There is a window for "buf" in the current tab page, make it the
@@ -1660,6 +1662,9 @@
 #ifdef FEAT_JOB_CHANNEL
 	// May need to stop Insert mode if we were in a prompt buffer.
 	leaving_window(curwin);
+	// Do not stop Insert mode when already in Insert mode before.
+	if (aco->save_State & MODE_INSERT)
+	    stop_insert_mode = FALSE;
 #endif
 	// Remove the window and frame from the tree of frames.
 	(void)winframe_remove(curwin, &dummy, NULL);