Nodes: Fix drag link from output to already linked Multi-Input Socket

This patch fixes a visual bug related to connecting an output socket to
a Multi-Input Socket, that already has a link to that same output.
In this case, the drag link got a new index and snapped to a new
position. This path makes the drag link snap to the same position as the
first link between the two sockets.

Differential Revision: https://developer.blender.org/D10689

1 files changed, 14 insertions, 0 deletions

diff --git a/source/blender/editors/space_node/node_relationships.c b/source/blender/editors/space_node/node_relationships.c
index 963349f876b..9293494a16a 100644
--- a/source/blender/editors/space_node/node_relationships.c
+++ b/source/blender/editors/space_node/node_relationships.c
@@ -890,10 +890,24 @@ static void node_link_find_socket(bContext *C, wmOperator *op, float cursor[2])
           continue;
         }
 
+        /* Skip if tsock is already linked with this output. */
+        bNodeLink *existing_link_connected_to_fromsock = NULL;
+        LISTBASE_FOREACH (bNodeLink *, existing_link, &snode->edittree->links) {
+          if (existing_link->fromsock == link->fromsock && existing_link->tosock == tsock) {
+            existing_link_connected_to_fromsock = existing_link;
+            break;
+          }
+        }
+
         /* attach links to the socket */
         link->tonode = tnode;
         link->tosock = tsock;
         snode->runtime->last_node_hovered_while_dragging_a_link = tnode;
+        if (existing_link_connected_to_fromsock) {
+          link->multi_input_socket_index =
+              existing_link_connected_to_fromsock->multi_input_socket_index;
+          continue;
+        }
         sort_multi_input_socket_links(snode, tnode, link, cursor);
       }
     }