Added .info.yml, .module, and LayoutBuilder element override class. Updated README.

README.md

+# Layout Builder Direct Add
+Converts the "Add block" link into a drop-button to directly add custom block types. The original "Add block" action is preserved as a "More..." link.
\ No newline at end of file

lb_direct_add.info.yml

+name: "Layout Builder Direct Add"
+description: Converts the "Add block" link into a drop-button to directly add custom block types.
+type: module
+core: 8.x
+dependencies:
+  - drupal:layout_builder
+  - drupal:block

lb_direct_add.module

+<?php
+
+/**
+ * @file
+ * Contains lb_direct_add.module.
+ */
+
+use Drupal\lb_direct_add\Element\LayoutBuilderDirectAdd;
+
+/**
+ * Implements hook_element_plugin_alter().
+ *
+ * Requires 8.8.x+ or this patch: https://www.drupal.org/files/issues/2019-07-15/2987208-19.patch.
+ */
+function lb_enhancements_element_plugin_alter(array &$definitions) {
+  $definitions['layout_builder']['class'] = LayoutBuilderDirectAdd::class;
+}

src/Element/LayoutBuilderDirectAdd.php

+<?php
+
+namespace Drupal\lb_direct_add\Element;
+
+use Drupal\Core\Render\Element;
+use Drupal\Core\Url;
+use Drupal\layout_builder\Element\LayoutBuilder;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * An override class to change the behavior of LayoutBuilder.
+ */
+class LayoutBuilderDirectAdd extends LayoutBuilder {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function buildAdministrativeSection(SectionStorageInterface $section_storage, $delta) {
+    $build = parent::buildAdministrativeSection($section_storage, $delta);
+    $storage_type = $section_storage->getStorageType();
+    $storage_id = $section_storage->getStorageId();
+    $section = $section_storage->getSection($delta);
+
+    $layout = $section->getLayout();
+    $layout_definition = $layout->getPluginDefinition();
+
+    $region_labels = $layout_definition->getRegionLabels();
+
+    foreach (Element::children($build['layout-builder__section']) as $region) {
+      $block_manager = \Drupal::getContainer()->get('plugin.manager.block');
+      $definitions = $block_manager->getFilteredDefinitions('layout_builder', $this->getAvailableContexts($section_storage), [
+        'section_storage' => $section_storage,
+        'region' => $region,
+        'list' => 'inline_blocks',
+      ]);
+      $blocks = $block_manager->getGroupedDefinitions($definitions);
+      $links = [];
+      $inline_blocks_category = (string) $this->t('Inline blocks');
+      if (isset($blocks[$inline_blocks_category])) {
+        foreach ($blocks[$inline_blocks_category] as $block_id => $block) {
+          $attributes = [
+            'class' => [
+              'use-ajax',
+              // 'layout-builder__link',
+              // 'layout-builder__link--add',
+            ],
+            'data-dialog-type' => 'dialog',
+            'data-dialog-renderer' => 'off_canvas',
+          ];
+          $attributes['class'][] = 'js-layout-builder-block-link';
+          $link = [
+            'title' => "Add {$block['admin_label']}",
+            'url' => Url::fromRoute('layout_builder.add_block',
+              [
+                'section_storage_type' => $section_storage->getStorageType(),
+                'section_storage' => $section_storage->getStorageId(),
+                'delta' => $delta,
+                'region' => $region,
+                'plugin_id' => $block_id,
+              ]
+            ),
+            'attributes' => $attributes,
+          ];
+
+          $links[] = $link;
+        }
+      }
+
+      $links['other'] = [
+        'title' => $this->t('More<span class="visually-hidden"> options</span>...'),
+        'url' => Url::fromRoute('layout_builder.choose_block',
+          [
+            'section_storage_type' => $storage_type,
+            'section_storage' => $storage_id,
+            'delta' => $delta,
+            'region' => $region,
+          ],
+          [
+            'attributes' => [
+              'class' => [
+                'use-ajax',
+                // 'layout-builder__link',
+                // 'layout-builder__link--add',
+              ],
+              'data-dialog-type' => 'dialog',
+              'data-dialog-renderer' => 'off_canvas',
+            ],
+          ]
+        ),
+      ];
+
+      $build['layout-builder__section'][$region]['layout_builder_add_block']['button'] = [
+        '#type' => 'dropbutton',
+        '#links' => $links,
+      ];
+
+      unset($build['layout-builder__section'][$region]['layout_builder_add_block']['link']);
+    }
+
+    return $build;
+  }
+
+}