fix: dragging past the bottom of the tree no longer slows down rendering

When dragging a node with a visible child to the last row displayed
for the tree, a render was triggered for each dragover event, causing
significant slowdown. The issue lied in the fact that the child was
being perceived as another node with which the dragged node should
swap with. The new implementation adds another condition - stopping the
rendering short if the previous render used the same drag position
(depth and minimum index).

Author: fritz-c

Diff for: src/react-sortable-tree.js

@@ -319,6 +319,14 @@ class ReactSortableTree extends Component {
     depth: draggedDepth,
     minimumTreeIndex: draggedMinimumTreeIndex,
   }) {
+    // Ignore this hover if it is at the same position as the last hover
+    if (
+      this.state.draggedDepth === draggedDepth &&
+      this.state.draggedMinimumTreeIndex === draggedMinimumTreeIndex
+    ) {
+      return;
+    }
+
     // Fall back to the tree data if something is being dragged in from
     //  an external element
     const draggingTreeData = this.state.draggingTreeData || this.props.treeData;