[FEATURE] Improve localization of pages

This patch implements two changes:

* Sets "allowLanguageSynchronization" behavior on page template fields.
* Copies values of "page configuration" fields when page is localized.

Close: FluidTYPO3#1910

Classes/Integration/HookSubscribers/DataHandlerSubscriber.php

@@ -12,6 +12,7 @@
 use FluidTYPO3\Flux\Content\ContentTypeManager;
 use FluidTYPO3\Flux\Provider\Interfaces\GridProviderInterface;
 use FluidTYPO3\Flux\Provider\Interfaces\RecordProcessingProvider;
+use FluidTYPO3\Flux\Provider\PageProvider;
 use FluidTYPO3\Flux\Provider\ProviderResolver;
 use FluidTYPO3\Flux\Utility\ColumnNumberUtility;
 use TYPO3\CMS\Backend\Utility\BackendUtility;
@@ -169,6 +170,26 @@ public function processDatamap_afterDatabaseOperations($command, $table, $id, $f
     // @phpcs:ignore PSR1.Methods.CamelCapsMethodName
     public function processDatamap_preProcessFieldArray(array &$fieldArray, $table, $id, DataHandler $dataHandler)
     {
+        if ($table === 'pages' && strpos((string) $id, 'NEW') === 0 && ($fieldArray['l10n_source'] ?? 0) > 0) {
+            // Record is a newly created page and is a translation of a page. In all likelyhood (but we can't actually
+            // know for sure since TYPO3 uses a nested DataHandler for this...) this record is the result of a blank
+            // initial copy of the original language's record. We may want to copy the "Page Configuration" fields'
+            // values from the original record.
+            if (!isset($fieldArray[PageProvider::FIELD_NAME_MAIN], $fieldArray[PageProvider::FIELD_NAME_SUB])) {
+                // To make completely sure, we only want to copy those values if both "Page Configuration" fields are
+                // completely omitted from the incoming field array.
+                $originalLanguageRecord = $this->getSingleRecordWithoutRestrictions(
+                    'pages',
+                    $fieldArray['l10n_source'],
+                    PageProvider::FIELD_NAME_MAIN . ',' . PageProvider::FIELD_NAME_SUB
+                );
+                if ($originalLanguageRecord) {
+                    $fieldArray[PageProvider::FIELD_NAME_MAIN] = $originalLanguageRecord[PageProvider::FIELD_NAME_MAIN];
+                    $fieldArray[PageProvider::FIELD_NAME_SUB] = $originalLanguageRecord[PageProvider::FIELD_NAME_SUB];
+                }
+            }
+        }
+
         // Handle "$table.$field" named fields where $table is the valid TCA table name and $field is an existing TCA
         // field. Updated value will still be subject to permission checks.
         $resolver = $this->getProviderResolver();

Configuration/TCA/Overrides/pages.php

@@ -12,6 +12,9 @@
             'config' => [
                 'type' => 'select',
                 'renderType' => 'selectSingle',
+                'behaviour' => [
+                    'allowLanguageSynchronization' => true,
+                ],
                 'fieldWizard' => [
                     'selectIcons' => [
                         'disabled' => false
@@ -23,9 +26,13 @@
             'exclude' => 1,
             'label' => 'LLL:EXT:flux/Resources/Private/Language/locallang.xlf:pages.tx_fed_page_controller_action_sub',
             'onChange' => 'reload',
+
             'config' => [
                 'type' => 'select',
                 'renderType' => 'selectSingle',
+                'behaviour' => [
+                    'allowLanguageSynchronization' => true,
+                ],
                 'fieldWizard' => [
                     'selectIcons' => [
                         'disabled' => false