Let the user sort layout plugins

That's cool, but we need strict test coverage.

The most important purpose of the weight is to move one item to the first place and thus make it the default.
We need to check that the button displays the plugins in the right order.
And we need to check that disabled plugins go away and are not accessible through the route.

Yeah then we can also split introducing weight stuff from the default as two separate issues.
In the worst case we can really also separately consider a/the default setting.

1. +++ b/src/DiffLayoutManager.php
   @@ -57,4 +57,55 @@ class DiffLayoutManager extends DefaultPluginManager {
   +   * Gets the applicable plugin options for a given field.
   ...
   +   * Loop over the plugins that can be applied to the field and builds an array
   +   * of possible plugins based on each plugin weight.
   

   What field?

2. +++ b/src/DiffLayoutManager.php
   @@ -57,4 +57,55 @@ class DiffLayoutManager extends DefaultPluginManager {
   +    $plugins = $this->config->get('general_settings' . '.' . 'layout_plugins');
   ...
   +    else {
   +      $plugins = $this->getDefinitions();
   +      foreach ($plugins as $key => $value) {
   +        $plugin_options[$key] = $value['label'];
   +      }
   

   We could simply force population of the settings on install time and then drop this else on runtime.
   Every line less here helps.

3. +++ b/src/DiffLayoutManager.php
   @@ -57,4 +57,55 @@ class DiffLayoutManager extends DefaultPluginManager {
   +    // If layout settings have been saved used them, otherwise load all the
   +    // plugins available.
   

   Not true.

4. +++ b/src/DiffLayoutManager.php
   @@ -57,4 +57,55 @@ class DiffLayoutManager extends DefaultPluginManager {
   +      // If any plugin is selected, all selected.
   +      if ($plugin_options == []) {
   +        $plugin_options = $plugins_order;
   

   I would skip this any=all part. It is only needed to have something grow if instances are created often. Make sure enabled is the default.

5. +++ b/src/Form/GeneralSettingsForm.php
   @@ -58,6 +58,57 @@ class GeneralSettingsForm extends ConfigFormBase {
   +        'enabled' => $config->get('general_settings' . '.' . 'layout_plugins')[$id]['enabled'] ?: FALSE,
   +        'weight' => $config->get('general_settings' . '.' . 'layout_plugins')[$id]['weight'] ?: $weight,
   

   Can't see where those are saved?

Getting the options reads straight forward now. So this seems fine.
Enabling all makes sense IMHO, it's how it is now.

What breaks if we disable classic layout?
Test fails are uncool. ;-)

Yeah then sure no default like that and make the method negotiate the default in code. That sounds pretty straight forward.

1. +++ b/src/Form/GeneralSettingsForm.php
   @@ -58,6 +86,57 @@ class GeneralSettingsForm extends ConfigFormBase {
   +    $layout_plugins = $this->diffLayoutManager->getDefinitions();
   +    $weight = count($layout_plugins) + 1;
   ...
   +        'weight' => $config->get('general_settings' . '.' . 'layout_plugins')[$id]['weight'] ?: $weight,
   

   Weights are hardcoded. You make the loop start at a position that depends the plugin count. That doesn't allow to hardcode things.

   BTW Isn't there a standard method that is called on submit that reorders weight?

2. +++ b/src/Tests/AdminFormsTest.php
   @@ -70,4 +70,71 @@ class AdminFormsTest extends DiffTestBase {
   +    // Create a node with a revision.
   +    $edit = [
   +      'title[0][value]' => 'great_title',
   +      'body[0][value]' => '<p>great_body</p>',
   +    ];
   +    $this->drupalPostForm('node/add/article', $edit, t('Save and publish'));
   +    $this->clickLink('Edit');
   +    $edit = [
   +      'title[0][value]' => 'greater_title',
   +      'body[0][value]' => '<p>greater_body</p>',
   +    ];
   +    $this->drupalPostForm(NULL, $edit, t('Save and keep published'));
   

   Hm, this reads so much repetitive, as if we need to do it again and again for every tiny test.
   Can you quickly check tests and if it makes sense to prepare this better?
   I think it belongs to the DiffTestBase.

3. +++ b/src/DiffLayoutManager.php
   @@ -57,4 +57,43 @@ class DiffLayoutManager extends DefaultPluginManager {
   +    list($plugin) = array_keys($plugins);
   +    return $plugin;
   

   You should use reset to pick the first.

OK simple solution:

1. +++ b/config/install/diff.settings.yml
   @@ -3,3 +3,13 @@ general_settings:
   +      weight: -50
   ...
   +      weight: -49
   ...
   +      weight: -48
   

   I'd start at 0.

2. +++ b/src/Form/GeneralSettingsForm.php
   @@ -58,6 +86,57 @@ class GeneralSettingsForm extends ConfigFormBase {
   +    $weight = count($layout_plugins) + 1;
   

   Let's take max() +1

OK committing this. But i'm not happy with count() as it can produce clashes when new plugins are added and custom weights are in place.