views_menu_link_defaults() does not set a parent for links

If we have a bug, we have a bug. we shouldn't be changing the UI to work around it.
I don't think this is a good approach.

Don't we still want to implement some magic to auto-set the parent based upon the path?

Maybe something like this?

+++ b/core/modules/views/lib/Drupal/views/Plugin/views/display/PathPluginBase.php
@@ -321,6 +333,26 @@ public function executeHookMenuLinkDefaults(array &$existing_links) {
+        if ($parent_paths) {
+          $mlids = $this->menuLinkStorage->getQuery()
+            ->condition('link_path', $parent_paths)
+            ->sort('link_path', 'DESC')
+            ->range(0, 1)
+            ->execute();
+          if ($mlids) {
+            $parent_menu_link = $this->menuLinkStorage->load(reset($mlids));
+            $links[$menu_link_id]['parent'] = $parent_menu_link->machine_name;
+          }

This will be problematic for a lot of reasons. We should use existing_links sadly, mh this is an N*M algorithm. The reason why we can't use it is that we are actually buillding menu links at the moment, which means that if at some points the menu links have to be actually rebuilt, good luck.
I will have a deeper look at this issue tomorrow.

+++ b/core/modules/views/lib/Drupal/views/Plugin/views/display/Page.php
@@ -216,32 +217,17 @@ public function buildOptionsForm(&$form, &$form_state) {
+            '#title' => t('Parent link'),
+            '#default_value' => $menu['menu_name'] . ':' . $menu['parent'],
+            '#options' => $options,

We kind of need to be able to select no parent.

On top of that some tests would be kinda cool.

Menu links are now plugins, this is still an issue and needs revisiting.

First attempt at a rework with menu link plugins. I could add a view display below an existing menu item and successfully edit it in either the menu UI or the Views UI in manual testing.

I will try to come up with some proper tests unless someone beets me until tomorrow.

1. +++ b/core/modules/views/src/Plugin/views/display/Page.php
   @@ -51,6 +51,7 @@ protected function defineOptions() {
   +        'parent' => array('default' => 0),
   

   The default is an empty plugin ID "", not 0 anymore.

2. +++ b/core/modules/views/src/Plugin/views/display/Page.php
   @@ -217,13 +218,14 @@ public function buildOptionsForm(&$form, FormStateInterface $form_state) {
   +            '#description' => t('The maximum depth for a link and all its children is fixed. Some menu links may not be available as parents if selecting them would exceed this limit.'),
   

   We could at least use $this->t()

3. +++ b/core/modules/views/src/Plugin/views/display/Page.php
   @@ -409,7 +412,9 @@ public function submitOptionsForm(&$form, FormStateInterface $form_state) {
   +        $menu = $form_state['values']['menu'];
   +        list($menu['name'], $menu['parent']) = explode(':', $menu['parent'], 2);
   +        $this->setOption('menu', $menu);
   

   Perfect!

Fixed #26.1 and #26.2.

Here is a test.

Surely pgae is a typo (twice) in that test?

Rebased and fixed test.

Urg, I hate myself.

This should be it.

           $form['menu']['menu_name'] = array(
             '#title' => t('Menu'),
             '#type' => 'select',
             '#options' => menu_ui_get_menus(),
             '#default_value' => $menu['menu_name'],
+          );
...
+        $menu = $form_state->getValue('menu');
+        list($menu['menu_name'], $menu['parent']) = explode(':', $menu['parent'], 2);

I think the Menu dropdown should be removed, the menu is selected via the Parent dropdown and the menu_name variable is overwritten at submit time anyway.

I think the Menu dropdown should be removed, the menu is selected via the Parent dropdown and the menu_name variable is overwritten at submit time anyway.

You are absolute right!

Not sure I am allowed to RTBC this as I worked on part of it, but I think #36 is good to go.

I really think that everyone in the community should be allowed to RTBC any issue, as long you are convinced that is the right thing and fixes the described problem.

Committed ce842f8 and pushed to 8.0.x. Thanks!