diff --git a/core/lib/Drupal/Core/Layout/LayoutPluginManager.php b/core/lib/Drupal/Core/Layout/LayoutPluginManager.php
index 8ff11eb..bc8b676 100644
--- a/core/lib/Drupal/Core/Layout/LayoutPluginManager.php
+++ b/core/lib/Drupal/Core/Layout/LayoutPluginManager.php
@@ -129,10 +129,7 @@ public function processDefinition(&$definition, $plugin_id) {
         $template_path .= '/' . implode('/', $template_parts);
       }
       $definition->setTemplate($template);
-      // Prepend 'layout__' so the base theme hook will be used.
-      // @todo Remove this workaround for https://www.drupal.org/node/2559825 in
-      //   https://www.drupal.org/node/2834019.
-      $definition->setThemeHook('layout__' . strtr($template, '-', '_'));
+      $definition->setThemeHook(strtr($template, '-', '_'));
       $definition->setTemplatePath($template_path);
     }
 
diff --git a/core/lib/Drupal/Core/Render/theme.api.php b/core/lib/Drupal/Core/Render/theme.api.php
index 0dd7573..2970b27 100644
--- a/core/lib/Drupal/Core/Render/theme.api.php
+++ b/core/lib/Drupal/Core/Render/theme.api.php
@@ -1138,7 +1138,7 @@ function hook_page_bottom(array &$page_bottom) {
  *     Instead of this suggestion's implementation being used directly, the base
  *     hook will be invoked with this implementation as its first suggestion.
  *     The base hook's files will be included and the base hook's preprocess
- *     functions will be called in place of any suggestion's preprocess
+ *     functions will be called in addition to any suggestion's preprocess
  *     functions. If an implementation of hook_theme_suggestions_HOOK() (where
  *     HOOK is the base hook) changes the suggestion order, a different
  *     suggestion may be used in place of this suggestion. If after
diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 0cb00a6..a3054e3 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -606,27 +606,56 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
   protected function completeSuggestion($hook, array &$cache) {
     $previous_hook = $hook;
     $incomplete_previous_hook = array();
+    // Continue looping if the candidate hook doesn't exist or if the candidate
+    // hook has incomplete preprocess functions, and if the candidate hook is a
+    // suggestion (has a double underscore).
     while ((!isset($cache[$previous_hook]) || isset($cache[$previous_hook]['incomplete preprocess functions']))
       && $pos = strrpos($previous_hook, '__')) {
+      // Find the first existing candidate hook that has incomplete preprocess
+      // functions.
       if (isset($cache[$previous_hook]) && !$incomplete_previous_hook && isset($cache[$previous_hook]['incomplete preprocess functions'])) {
         $incomplete_previous_hook = $cache[$previous_hook];
         unset($incomplete_previous_hook['incomplete preprocess functions']);
       }
       $previous_hook = substr($previous_hook, 0, $pos);
+      $this->mergePreprocessFunctions($hook, $previous_hook, $incomplete_previous_hook, $cache);
+    }
 
-      // If base hook exists clone of it for the preprocess function
-      // without a template.
-      // @see https://www.drupal.org/node/2457295
-      if (isset($cache[$previous_hook]) && !isset($cache[$previous_hook]['incomplete preprocess functions'])) {
-        $cache[$hook] = $incomplete_previous_hook + $cache[$previous_hook];
-        if (isset($incomplete_previous_hook['preprocess functions'])) {
-          $diff = array_diff($incomplete_previous_hook['preprocess functions'], $cache[$previous_hook]['preprocess functions']);
-          $cache[$hook]['preprocess functions'] = array_merge($cache[$previous_hook]['preprocess functions'], $diff);
-        }
-        // If a base hook isn't set, this is the actual base hook.
-        if (!isset($cache[$previous_hook]['base hook'])) {
-          $cache[$hook]['base hook'] = $previous_hook;
-        }
+    // In addition to processing suggestions, include base hooks.
+    if (isset($cache[$hook]['base hook'])) {
+      // In order to retain the additions from above, pass in the current hook
+      // as the parent hook, otherwise it will be overwritten.
+      $this->mergePreprocessFunctions($hook, $cache[$hook]['base hook'], $cache[$hook], $cache);
+    }
+  }
+
+  /**
+   * Merges the source hook's preprocess functions into the destination hook's.
+   *
+   * @param string $destination_hook_name
+   *   The name of the hook to merge preprocess functions to.
+   * @param string $source_hook_name
+   *   The name of the hook to merge preprocess functions from.
+   * @param array $parent_hook
+   *   The parent hook if it exists. Either an incomplete hook from suggestions
+   *   or a base hook.
+   * @param array $cache
+   *   The theme registry, as documented in
+   *   \Drupal\Core\Theme\Registry::processExtension().
+   */
+  protected function mergePreprocessFunctions($destination_hook_name, $source_hook_name, $parent_hook, array &$cache) {
+    // If base hook exists clone of it for the preprocess function
+    // without a template.
+    // @see https://www.drupal.org/node/2457295
+    if (isset($cache[$source_hook_name]) && (!isset($cache[$source_hook_name]['incomplete preprocess functions']) || !isset($cache[$destination_hook_name]['incomplete preprocess functions']))) {
+      $cache[$destination_hook_name] = $parent_hook + $cache[$source_hook_name];
+      if (isset($parent_hook['preprocess functions'])) {
+        $diff = array_diff($parent_hook['preprocess functions'], $cache[$source_hook_name]['preprocess functions']);
+        $cache[$destination_hook_name]['preprocess functions'] = array_merge($cache[$source_hook_name]['preprocess functions'], $diff);
+      }
+      // If a base hook isn't set, this is the actual base hook.
+      if (!isset($cache[$source_hook_name]['base hook'])) {
+        $cache[$destination_hook_name]['base hook'] = $source_hook_name;
       }
     }
   }
diff --git a/core/tests/Drupal/Tests/Core/Layout/LayoutPluginManagerTest.php b/core/tests/Drupal/Tests/Core/Layout/LayoutPluginManagerTest.php
index d892b47..2d89859 100644
--- a/core/tests/Drupal/Tests/Core/Layout/LayoutPluginManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Layout/LayoutPluginManagerTest.php
@@ -119,7 +119,7 @@ public function testGetDefinition() {
     $this->assertSame('twocol', $layout_definition->getTemplate());
     $this->assertSame("$theme_a_path/templates", $layout_definition->getPath());
     $this->assertSame('theme_a/twocol', $layout_definition->getLibrary());
-    $this->assertSame('layout__twocol', $layout_definition->getThemeHook());
+    $this->assertSame('twocol', $layout_definition->getThemeHook());
     $this->assertSame("$theme_a_path/templates", $layout_definition->getTemplatePath());
     $this->assertSame('theme_a', $layout_definition->getProvider());
     $this->assertSame('right', $layout_definition->getDefaultRegion());
@@ -165,7 +165,7 @@ public function testGetDefinition() {
     $this->assertSame('plugin-provided-layout', $layout_definition->getTemplate());
     $this->assertSame($core_path, $layout_definition->getPath());
     $this->assertSame(NULL, $layout_definition->getLibrary());
-    $this->assertSame('layout__plugin_provided_layout', $layout_definition->getThemeHook());
+    $this->assertSame('plugin_provided_layout', $layout_definition->getThemeHook());
     $this->assertSame("$core_path/templates", $layout_definition->getTemplatePath());
     $this->assertSame('core', $layout_definition->getProvider());
     $this->assertSame('main', $layout_definition->getDefaultRegion());
@@ -209,13 +209,13 @@ public function testGetThemeImplementations() {
       'layout' => [
         'render element' => 'content',
       ],
-      'layout__twocol' => [
+      'twocol' => [
         'render element' => 'content',
         'base hook' => 'layout',
         'template' => 'twocol',
         'path' => "$theme_a_path/templates",
       ],
-      'layout__plugin_provided_layout' => [
+      'plugin_provided_layout' => [
         'render element' => 'content',
         'base hook' => 'layout',
         'template' => 'plugin-provided-layout',
diff --git a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
index 1243118..8be62d8 100644
--- a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
+++ b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
@@ -67,6 +67,13 @@ class RegistryTest extends UnitTestCase {
   protected $themeManager;
 
   /**
+   * The list of functions that get_defined_functions() should provide.
+   *
+   * @var array
+   */
+  public static $functions = [];
+
+  /**
    * {@inheritdoc}
    */
   protected function setUp() {
@@ -83,6 +90,14 @@ protected function setUp() {
   }
 
   /**
+   * {@inheritdoc}
+   */
+  protected function tearDown() {
+    parent::tearDown();
+    static::$functions = [];
+  }
+
+  /**
    * Tests getting the theme registry defined by a module.
    */
   public function testGetRegistryForModule() {
@@ -159,6 +174,317 @@ public function testGetRegistryForModule() {
     $this->assertTrue(in_array('test_stable_preprocess_theme_test_render_element', $other_registry['theme_test_render_element']['preprocess functions']));
   }
 
+  /**
+   * @covers ::postProcessExtension
+   * @covers ::completeSuggestion
+   * @covers ::mergePreprocessFunctions
+   *
+   * @dataProvider providerTestPostProcessExtension
+   *
+   * @param array $defined_functions
+   *   An array of functions to be used in place of get_defined_functions().
+   * @param array $hooks
+   *   An array of theme hooks to process.
+   * @param array $expected
+   *   The expected results.
+   */
+  public function testPostProcessExtension($defined_functions, $hooks, $expected) {
+    static::$functions['user'] = $defined_functions;
+
+    $theme = $this->prophesize(ActiveTheme::class);
+    $theme->getBaseThemes()->willReturn([]);
+    $theme->getName()->willReturn('test');
+    $theme->getEngine()->willReturn('twig');
+
+    $this->moduleHandler->expects($this->atLeastOnce())
+      ->method('getModuleList')
+      ->willReturn([]);
+
+    $class = new \ReflectionClass(TestRegistry::class);
+    $reflection_method = $class->getMethod('postProcessExtension');
+    $reflection_method->setAccessible(TRUE);
+    $reflection_method->invokeArgs($this->registry, [&$hooks, $theme->reveal()]);
+
+    $this->assertArrayEquals($expected, $hooks);
+  }
+
+  /**
+   * Provides test data to ::testPostProcessExtension().
+   */
+  public function providerTestPostProcessExtension() {
+    // This is test data for unit testing
+    // \Drupal\Core\Theme\Registry::postProcessExtension(), not what happens
+    // before it. Therefore, for all test data:
+    // - Explicitly defined hooks also come with explicitly defined preprocess
+    //   functions, because those are collected in
+    //   \Drupal\Core\Theme\Registry::processExtension().
+    // - Explicitly defined hooks that set a 'base hook' also have
+    //   'incomplete preprocess functions' set to TRUE, since that is done in
+    //   \Drupal\Core\Theme\Registry::processExtension().
+    $data = [];
+
+    // Test the discovery of suggestions via the presence of preprocess
+    // functions that follow the "__" naming pattern.
+    $data['base_hook_with_autodiscovered_suggestions'] = [
+      'defined_functions' => [
+        'test_preprocess_test_hook__suggestion',
+        'test_preprocess_test_hook__suggestion__another',
+      ],
+      'hooks' => [
+        'test_hook' => [
+          'preprocess functions' => ['explicit_preprocess_test_hook'],
+        ],
+      ],
+      'expected' => [
+        'test_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+          ],
+        ],
+        'test_hook__suggestion' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'test_preprocess_test_hook__suggestion',
+          ],
+          'base hook' => 'test_hook',
+        ],
+        'test_hook__suggestion__another' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'test_preprocess_test_hook__suggestion',
+            'test_preprocess_test_hook__suggestion__another',
+          ],
+          'base hook' => 'test_hook',
+        ],
+      ],
+    ];
+
+    // Test that suggestions following the "__" naming pattern can also be
+    // explicitly defined in hook_theme(), such as 'field__node__title' defined
+    // in node_theme().
+    $data['base_hook_with_explicit_suggestions'] = [
+      'defined_functions' => [],
+      'hooks' => [
+        'test_hook' => [
+          'preprocess functions' => ['explicit_preprocess_test_hook'],
+        ],
+        'test_hook__suggestion__another' => [
+          'base hook' => 'test_hook',
+          'preprocess functions' => ['explicit_preprocess_test_hook__suggestion__another'],
+          'incomplete preprocess functions' => TRUE,
+        ],
+      ],
+      'expected' => [
+        'test_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+          ],
+        ],
+        'test_hook__suggestion__another' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'explicit_preprocess_test_hook__suggestion__another',
+          ],
+          'base hook' => 'test_hook',
+        ],
+      ],
+    ];
+
+    // Same as above, but also test that a preprocess function for an
+    // intermediary suggestion level gets discovered.
+    $data['base_hook_with_explicit_suggestions_and_intermediary_preprocess_function'] = [
+      'defined_functions' => [
+        'test_preprocess_test_hook__suggestion',
+      ],
+      'hooks' => [
+        'test_hook' => [
+          'preprocess functions' => ['explicit_preprocess_test_hook'],
+        ],
+        'test_hook__suggestion__another' => [
+          'base hook' => 'test_hook',
+          'preprocess functions' => ['explicit_preprocess_test_hook__suggestion__another'],
+          'incomplete preprocess functions' => TRUE,
+        ],
+      ],
+      'expected' => [
+        'test_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+          ],
+        ],
+        'test_hook__suggestion' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'test_preprocess_test_hook__suggestion',
+          ],
+          'base hook' => 'test_hook',
+        ],
+        'test_hook__suggestion__another' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'test_preprocess_test_hook__suggestion',
+            'explicit_preprocess_test_hook__suggestion__another',
+          ],
+          'base hook' => 'test_hook',
+        ],
+      ],
+    ];
+
+    // Test that hooks not following the "__" naming pattern can explicitly
+    // specify a base hook, such as is done in
+    // \Drupal\Core\Layout\LayoutPluginManager::getThemeImplementations().
+    $data['child_hook_without_magic_naming'] = [
+      'defined_functions' => [],
+      'hooks' => [
+        'test_hook' => [
+          'preprocess functions' => ['explicit_preprocess_test_hook'],
+        ],
+        'child_hook' => [
+          'base hook' => 'test_hook',
+          'preprocess functions' => ['explicit_preprocess_child_hook'],
+          'incomplete preprocess functions' => TRUE,
+        ],
+      ],
+      'expected' => [
+        'test_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+          ],
+        ],
+        'child_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'explicit_preprocess_child_hook',
+          ],
+          'base hook' => 'test_hook',
+        ],
+      ],
+    ];
+
+    // Same as above, but also test that such child hooks can also be extended
+    // with magically named suggestions.
+    $data['child_hook_with_suggestions'] = [
+      'defined_functions' => [
+        'test_preprocess_child_hook__suggestion',
+        'test_preprocess_child_hook__suggestion__another',
+      ],
+      'hooks' => [
+        'test_hook' => [
+          'preprocess functions' => ['explicit_preprocess_test_hook'],
+        ],
+        'child_hook' => [
+          'base hook' => 'test_hook',
+          'preprocess functions' => ['explicit_preprocess_child_hook'],
+          'incomplete preprocess functions' => TRUE,
+        ],
+      ],
+      'expected' => [
+        'test_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+          ],
+        ],
+        'child_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'explicit_preprocess_child_hook',
+          ],
+          'base hook' => 'test_hook',
+        ],
+        'child_hook__suggestion' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'explicit_preprocess_child_hook',
+            'test_preprocess_child_hook__suggestion',
+          ],
+          'base hook' => 'test_hook',
+        ],
+        'child_hook__suggestion__another' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'explicit_preprocess_child_hook',
+            'test_preprocess_child_hook__suggestion',
+            'test_preprocess_child_hook__suggestion__another',
+          ],
+          'base hook' => 'test_hook',
+        ],
+      ],
+    ];
+
+    // Test that a suggestion following the "__" naming pattern can specify a
+    // different base hook than what is implied by that pattern. Ensure that
+    // preprocess functions from both the naming pattern and from 'base hook'
+    // are collected.
+    $data['suggestion_with_alternate_base_hook'] = [
+      'defined_functions' => [
+        'test_preprocess_test_hook__suggestion',
+      ],
+      'hooks' => [
+        'test_hook' => [
+          'preprocess functions' => ['explicit_preprocess_test_hook'],
+        ],
+        'alternate_base_hook' => [
+          'preprocess functions' => ['explicit_preprocess_alternate_base_hook'],
+        ],
+        'test_hook__suggestion__another' => [
+          'base hook' => 'alternate_base_hook',
+          'preprocess functions' => ['explicit_preprocess_test_hook__suggestion__another'],
+          'incomplete preprocess functions' => TRUE,
+        ],
+      ],
+      'expected' => [
+        'test_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+          ],
+        ],
+        'alternate_base_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_alternate_base_hook',
+          ],
+        ],
+        'test_hook__suggestion' => [
+          'preprocess functions' => [
+            'explicit_preprocess_test_hook',
+            'test_preprocess_test_hook__suggestion',
+          ],
+          'base hook' => 'test_hook',
+        ],
+        'test_hook__suggestion__another' => [
+          'preprocess functions' => [
+            'explicit_preprocess_alternate_base_hook',
+            'explicit_preprocess_test_hook',
+            'test_preprocess_test_hook__suggestion',
+            'explicit_preprocess_test_hook__suggestion__another',
+          ],
+          'base hook' => 'alternate_base_hook',
+        ],
+      ],
+    ];
+
+    // Test when a base hook is missing.
+    $data['missing_base_hook'] = [
+      'defined_functions' => [],
+      'hooks' => [
+        'child_hook' => [
+          'base hook' => 'test_hook',
+          'preprocess functions' => ['explicit_preprocess_child_hook'],
+          'incomplete preprocess functions' => TRUE,
+        ],
+      ],
+      'expected' => [
+        'child_hook' => [
+          'preprocess functions' => [
+            'explicit_preprocess_child_hook',
+          ],
+          'base hook' => 'test_hook',
+        ],
+      ],
+    ];
+
+    return $data;
+  }
+
   protected function setupTheme() {
     $this->registry = new TestRegistry($this->root, $this->cache, $this->lock, $this->moduleHandler, $this->themeHandler, $this->themeInitialization);
     $this->registry->setThemeManager($this->themeManager);
@@ -175,3 +501,14 @@ protected function getPath($module) {
   }
 
 }
+
+namespace Drupal\Core\Theme;
+
+use Drupal\Tests\Core\Theme\RegistryTest;
+
+/**
+ * Overrides get_defined_functions() with a configurable mock.
+ */
+function get_defined_functions() {
+  return RegistryTest::$functions ?: \get_defined_functions();
+}