Error vs throw if the insert after el was no longer on the stack

Fixes #2393

Author: jhy

CHANGES.md

@@ -6,6 +6,7 @@
 * Previously cached child Elements of an Element were not correctly invalidated in `Node#replaceWith(Node)`, which could lead to incorrect results when subsequently calling `Element#children()`. [#2391](https://github.com/jhy/jsoup/issues/2391)
 * Attribute selector values are now compared literally without trimming. Previously, jsoup trimmed whitespace from selector values and from element attribute values, which could cause mismatches with browser behavior (e.g. `[attr=" foo "]`). Now matches align with the CSS specification and browser engines. [#2380](https://github.com/jhy/jsoup/issues/2380)
 * When using the JDK HttpClient, any system default proxy (`ProxySelector.getDefault()`) was ignored. Now, the system proxy is used if a per-request proxy is not set. [#2388](https://github.com/jhy/jsoup/issues/2388), [#2390](https://github.com/jhy/jsoup/pull/2390)
+* A ValidationException could be thrown in the adoption agency algorithm with particularly broken input. Now logged as a parse error. [#2393](https://github.com/jhy/jsoup/issues/2393)
 
 
 ## 1.21.2 (2025-Aug-25)

src/main/java/org/jsoup/parser/HtmlTreeBuilder.java

@@ -558,8 +558,13 @@ private void clearStackToContext(String... nodeNames) {
 
     void insertOnStackAfter(Element after, Element in) {
         int i = stack.lastIndexOf(after);
-        Validate.isTrue(i != -1);
-        stack.add(i+1, in);
+        if (i == -1) {
+            error("Did not find element on stack to insert after");
+            stack.add(in);
+            // may happen on particularly malformed inputs during adoption
+        } else {
+            stack.add(i+1, in);
+        }
     }
 
     void replaceOnStack(Element out, Element in) {