Fix proposal for #3435910

src/ConfigSplitManager.php

@@ -409,6 +409,34 @@ public function splitPreview(ImmutableConfig $config, StorageInterface $transfor
           }
         }
       }
+
+      // Handle the case where some config entities have to exist in default
+      // config but not in splitted one.
+      // We get the collection names from default config, as some collections
+      // could be totally emptied in split config, thus not found by
+      // DatabaseStorage::getAllCollectionNames().
+      foreach (array_merge(
+                 [StorageInterface::DEFAULT_COLLECTION],
+                 $this->sync->getAllCollectionNames()
+               ) as $collection) {
+        $syncCollection = $preparedSync->createCollection($collection);
+        $sourceCollection = $source->createCollection($collection);
+        $storageCollection = $transforming->createCollection($collection);
+        $splitCollection = $splitStorage->createCollection($collection);
+
+        $partialList = array_filter($syncCollection->listAll(), function ($name) use ($partialSplitList, $completelySplit) {
+          // Check for wildcards. But skip config which is already split.
+          return !in_array($name, $completelySplit) && self::inFilterList($name, $partialSplitList);
+        });
+
+        foreach ($partialList as $name) {
+          if (!$sourceCollection->exists($name) && $syncCollection->exists($name)) {
+            $sync = $syncCollection->read($name);
+            $this->createPatch($name, [FALSE], $sync, $splitCollection);
+            $storageCollection->write($name, $sync);
+          }
+        }
+      }
     }
 
     // Now special case the extensions.
@@ -449,9 +477,17 @@ public function mergeSplit(ImmutableConfig $config, StorageInterface $transformi
             $diff = ConfigPatch::fromArray($data);
             if ($storage->exists($name)) {
               // Skip patches for config that doesn't exist in the storage.
-              $data = $storage->read($name);
-              $data = $this->patchMerge->mergePatch($data, $diff->invert(), $name);
-              $storage->write($name, $data);
+              if ((($diff->getRemoved()[0]) ?? NULL) === FALSE) {
+                // It means that this config entity which exists by default
+                // should not exist when the split is enabled.
+                // @see \Drupal\config_split\ConfigSplitManager::splitPreview().
+                $storage->delete($name);
+              }
+              else {
+                $data = $storage->read($name);
+                $data = $this->patchMerge->mergePatch($data, $diff->invert(), $name);
+                $storage->write($name, $data);
+              }
             }
           }
           else {