Ensure root is set early (#4184)

Closes #4177.

When a child view was removed from the DOM in between in joining and
receving the join callback, we'd fail to mark it as destroyed, because
destroyViewByEl uses the root id for lookup. This meant that the child
view crashed when trying to attach to the element.

Author: SteffenDE

assets/js/phoenix_live_view/view.js

@@ -110,6 +110,11 @@ export default class View {
     // bind the view to the element
     DOM.putPrivate(this.el, "view", this);
     this.id = this.el.id;
+    // destroyViewByEl requires the root set, so we need to set it early
+    // otherwise it could happen that we try to apply a join result for a
+    // view whose DOM node was already removed
+    // See https://github.com/phoenixframework/phoenix_live_view/issues/4177.
+    this.el.setAttribute(PHX_ROOT_ID, this.root.id);
     this.ref = 0;
     this.lastAckRef = null;
     this.childJoins = 0;