Network: Block recalculation of level in LayoutEngine._determineLevelsDirected() (#3294)

* Levels of direct hierarchical network only incremented

* Cleaned up old code

* Quick fix on presence  globaOptions in mergeOptions()

* Revert fix, doesn't work

* Network: Block recalculation of level in LayoutEngine._determineLevelsDirected()

Fix for #2311.

Nodes with bidirectional edges got their levels shifted due to the handling of both edge directions.
This fix adds a check on bidirectionality and blocks any subsequent level adjustment.
Pure tree layouts are unaffected by this change.

lib/network/modules/LayoutEngine.js

@@ -1414,7 +1414,7 @@ class LayoutEngine {
   }
 
   /**
-   * this function allocates nodes in levels based on the direction of the edges
+   * Allocate nodes in levels based on the direction of the edges.
    *
    * @param hubsize
    * @private
@@ -1422,8 +1422,30 @@ class LayoutEngine {
   _determineLevelsDirected() {
     let minLevel = 10000;
 
+    /**
+     * Check if there is an edge going the opposite direction for given edge
+     */
+    let self = this;
+    let isBidirectional = (edge) => {
+      for (let key in self.body.edges) {
+        let otherEdge = self.body.edges[key];
+        if (otherEdge.toId === edge.fromId && otherEdge.fromId === edge.toId) {
+          return true;
+        }
+      }
+
+      return false;
+    };
+
     let levelByDirection = (nodeA, nodeB, edge) => {
       let levelA = this.hierarchical.levels[nodeA.id];
+      let levelB = this.hierarchical.levels[nodeB.id];
+
+      if (isBidirectional(edge) && levelA !== undefined && levelB !== undefined) {
+        // Don't redo the level determination if already done in this case.
+        return;
+      }
+
       // set initial level
       if (levelA === undefined) { levelA = this.hierarchical.levels[nodeA.id] = minLevel;}
       if (edge.toId == nodeB.id) {