Fix adding nodes to nested fragments (#4929)

* Fix adding nodes to nested fragments

* add note about test

packages/core/src/diff/node.rs

@@ -188,10 +188,10 @@ impl VNode {
             Some((idx, Placeholder(_) | Text(_))) => {
                 ElementId(dom.get_mounted_dyn_node(mount_id, idx))
             }
-            // The node is a fragment, so we need to find the first element in the fragment
+            // The node is a fragment, so we need to find the last element in the fragment
             Some((_, Fragment(children))) => {
-                let child = children.first().unwrap();
-                child.find_first_element(dom)
+                let child = children.last().unwrap();
+                child.find_last_element(dom)
             }
             // The node is a component, so we need to find the first element in the component
             Some((id, Component(_))) => {

packages/core/tests/diff_unkeyed_list.rs

@@ -509,3 +509,83 @@ fn replace_and_add_items() {
         );
     }
 }
+
+// Simplified regression test for https://github.com/DioxusLabs/dioxus/issues/4924
+#[test]
+fn nested_unkeyed_lists() {
+    let mut dom = VirtualDom::new(|| {
+        let content = if generation() % 2 == 0 {
+            vec!["5\n6"]
+        } else {
+            vec!["1\n2", "3\n4"]
+        };
+
+        rsx! {
+            for one in &content {
+                for line in one.lines() {
+                    p { "{line}" }
+                }
+            }
+        }
+    });
+
+    // The list starts with one placeholder
+    {
+        let edits = dom.rebuild_to_vec();
+        assert_eq!(
+            edits.edits,
+            [
+                // load the p tag template
+                LoadTemplate { index: 0, id: ElementId(1) },
+                // Create the first text node
+                CreateTextNode { value: "5".into(), id: ElementId(2) },
+                // Replace the placeholder inside the p tag with the text node
+                ReplacePlaceholder { path: &[0], m: 1 },
+                // load the p tag template
+                LoadTemplate { index: 0, id: ElementId(3) },
+                // Create the second text node
+                CreateTextNode { value: "6".into(), id: ElementId(4) },
+                // Replace the placeholder inside the p tag with the text node
+                ReplacePlaceholder { path: &[0], m: 1 },
+                // Add the text nodes to the root node
+                AppendChildren { id: ElementId(0), m: 2 }
+            ]
+        );
+    }
+
+    // DOM state:
+    // <pre> # Id 1 for if statement
+    // <p> # Id 2
+    //    "5" # Id 3
+    // <p> # Id 4
+    //    "6" # Id 5
+    //
+    // The diffing engine should add two new elements to the end and modify the first two elements in place
+    {
+        dom.mark_dirty(ScopeId::APP);
+        let edits = dom.render_immediate_to_vec();
+        assert_eq!(
+            edits.edits,
+            [
+                // load the p tag template
+                LoadTemplate { index: 0, id: ElementId(5) },
+                // Create the third text node
+                CreateTextNode { value: "3".into(), id: ElementId(6) },
+                // Replace the placeholder inside the p tag with the text node
+                ReplacePlaceholder { path: &[0], m: 1 },
+                // load the p tag template
+                LoadTemplate { index: 0, id: ElementId(7) },
+                // Create the fourth text node
+                CreateTextNode { value: "4".into(), id: ElementId(8) },
+                // Replace the placeholder inside the p tag with the text node
+                ReplacePlaceholder { path: &[0], m: 1 },
+                // Insert the text nodes after the second p tag
+                InsertAfter { id: ElementId(3), m: 2 },
+                // Set the first text node to "1"
+                SetText { value: "1".into(), id: ElementId(2) },
+                // Set the second text node to "2"
+                SetText { value: "2".into(), id: ElementId(4) }
+            ]
+        );
+    }
+}