[DREAM-808] Fixes inserted checked node form state

https://community.openproject.org/wp/DREAM-808

Extends the TreeView form observer guard to recognize childList
mutations that add checked tree items. Previously only aria-checked
attribute changes triggered a rebuild of the hidden form inputs, so
checked nodes inserted after connect (e.g. from async fragments)
appeared selected but were omitted from form submission.

Adds a browser regression test cloning a checked node into an
already-connected tree.

Author: myabc

app/components/primer/alpha/tree_view/tree_view.ts

@@ -38,6 +38,17 @@ export class TreeViewElement extends HTMLElement {
       // nodes based on the component's select strategy. These two observers can conflict and cause infinite
       // looping, so we make sure something actually changed before computing inputs again.
       const somethingChanged = mutations.some(m => {
+        if (m.type === 'childList') {
+          return [...m.addedNodes].some(node => {
+            if (!(node instanceof Element)) return false
+
+            return (
+              node.matches('[role=treeitem][aria-checked=true]') ||
+              Boolean(node.querySelector('[role=treeitem][aria-checked=true]'))
+            )
+          })
+        }
+
         if (!(m.target instanceof HTMLElement)) return false
         return m.target.getAttribute('aria-checked') !== m.oldValue
       })

test/system/alpha/tree_view_test.rb

@@ -952,6 +952,32 @@ def test_initial_form_state
       assert_equal "{\"path\":[\"src\",\"button.rb\"],\"nodeId\":\"src-button-rb\",\"value\":\"1\"}", response.dig("form_params", "folder_structure", 0)
     end
 
+    def test_form_state_updates_when_checked_nodes_are_inserted
+      visit_preview(:form_input, expanded: true, route_format: :json)
+
+      evaluate_multiline_script(<<~JS)
+        const tree = document.querySelector("tree-view")
+        const checkedNode = tree.querySelector("[role=treeitem][aria-checked=true]")
+        const clone = checkedNode.closest("li").cloneNode(true)
+        const clonedTreeItem = clone.querySelector("[role=treeitem]")
+
+        clonedTreeItem.removeAttribute("id")
+        clonedTreeItem.removeAttribute("data-node-id")
+        clonedTreeItem.removeAttribute("data-value")
+        clonedTreeItem.setAttribute("data-path", JSON.stringify(["async.rb"]))
+
+        tree.querySelector(":scope > ul").append(clone)
+      JS
+
+      assert_selector("[data-target='tree-view.formInputContainer'] input", count: 2, visible: :all)
+
+      find("button[type=submit]").click
+
+      response = JSON.parse(find("pre").text)
+
+      assert_includes response.dig("form_params", "folder_structure"), "{\"path\":[\"async.rb\"]}"
+    end
+
     def test_form_submission_with_single_select_variant
       visit_preview(:form_input, expanded: true, select_variant: :single, route_format: :json)