More efficient update of Selection.isBackward

src/models/node.js

@@ -470,6 +470,31 @@ const Node = {
     return descendant
   },
 
+  /**
+   * True if the node has both descendants in that order, false
+   * otherwise. The order is depth-first, post-order.
+   *
+   * @param {String} key1
+   * @param {String} key2
+   * @return {Boolean} True if nodes are found in this order
+   */
+
+  areDescendantSorted(key1, key2) {
+    let sorted
+
+    this.forEachDescendant(n => {
+      if (n.key === key1) {
+        sorted = true
+        return false
+      } else if (n.key === key2) {
+        sorted = false
+        return false
+      }
+    })
+
+    return sorted
+  },
+
   /**
    * Get the depth of a child node by `key`, with optional `startAt`.
    *

src/models/selection.js

@@ -332,12 +332,10 @@ class Selection extends new Record(DEFAULTS) {
 
     // If `isBackward` is not set, derive it.
     if (isBackward == null) {
-      let texts = node.getTexts()
-      let anchorIndex = texts.indexOf(anchorNode)
-      let focusIndex = texts.indexOf(focusNode)
-      isBackward = anchorIndex == focusIndex
-        ? anchorOffset > focusOffset
-        : anchorIndex > focusIndex
+      if (anchorKey === focusKey) {
+        isBackward = anchorOffset > focusOffset
+      } else {
+        isBackward = !node.areDescendantSorted(anchorKey, focusKey)
     }
 
     // Merge in any updated properties.