Clear child nodes' parent on parent.empty()

Fixes #2013

CHANGES

@@ -64,6 +64,11 @@ Release 1.16.2 [PENDING]
     Further, a StackOverflowError may occur when running the query.
     <https://github.com/jhy/jsoup/issues/2001>
 
+  * Bugfix: appending a node back to its original Element after empty() would throw an Index out of bounds exception.
+    Also, now the child nodes that were removed have their parent node cleared, fully detaching them from the original
+    parent.
+    <https://github.com/jhy/jsoup/issues/2013>
+
   * Change: removed previously deprecated methods Document#normalise, Element#forEach(org.jsoup.helper.Consumer<>),
     Node#forEach(org.jsoup.helper.Consumer<>), and the org.jsoup.helper.Consumer interface; the latter being a
     previously required compatibility shim prior to Android's de-sugaring support.

src/main/java/org/jsoup/nodes/Element.java

@@ -829,11 +829,16 @@ public Element after(Node node) {
     }
 
     /**
-     * Remove all the element's child nodes. Any attributes are left as-is.
+     * Remove all the element's child nodes. Any attributes are left as-is. Each child node has its parent set to
+     * {@code null}.
      * @return this element
      */
     @Override
     public Element empty() {
+        // Detach each of the children -> parent links:
+        for (Node child : childNodes) {
+            child.parentNode = null;
+        }
         childNodes.clear();
         return this;
     }

src/test/java/org/jsoup/nodes/ElementTest.java

@@ -2773,4 +2773,47 @@ void prettySerializationRoundTrips(Document.OutputSettings settings) {
             "</table>", out);
         // todo - I would prefer the </td> to wrap down there - but need to reimplement pretty printer to simplify and track indented state
     }
+
+    @Test void emptyDetachesChildren() {
+        String html = "<div><p>One<p>Two</p>Three</div>";
+        Document doc = Jsoup.parse(html);
+        Element div = doc.expectFirst("div");
+        assertEquals(3, div.childNodeSize());
+
+        List<Node> childNodes = div.childNodes();
+
+        div.empty();
+        assertEquals(0, div.childNodeSize());
+        assertEquals(3, childNodes.size()); // copied before removing
+        for (Node childNode : childNodes) {
+            assertNull(childNode.parentNode);
+        }
+
+        Element p = (Element) childNodes.get(0);
+        assertEquals(p, p.childNode(0).parentNode()); // TextNode "One" still has parent p, as detachment is only on div element
+    }
+
+    @Test void emptyAndAddPreviousChild() {
+        String html = "<div><p>One<p>Two<p>Three</div>";
+        Document doc = Jsoup.parse(html);
+        Element div = doc.expectFirst("div");
+        Element p = div.expectFirst("p");
+        div
+            .empty()
+            .appendChild(p);
+
+        assertEquals("<p>One</p>", div.html());
+    }
+
+    @Test void emptyAndAddPreviousDescendant() {
+        String html = "<header><div><p>One<p>Two<p>Three</div></header>";
+        Document doc = Jsoup.parse(html);
+        Element header = doc.expectFirst("header");
+        Element p = header.expectFirst("p");
+        header
+            .empty()
+            .appendChild(p);
+
+        assertEquals("<p>One</p>", header.html());
+    }
 }