Feature: #99976 - Introduce ignoreFlexFormSettingsIfEmpty Extbase configuration

Description

It is now possible to exclude empty FlexForm settings from being merged into Extbase extension settings. Extension authors and integrators can use the new Extbase TypoScript configuration ignoreFlexFormSettingsIfEmpty to define FlexForm settings, which will be ignored in the merge process of the extension settings, if their value is considered empty (either an empty string or a string containing 0).

In the following example, settings.showForgotPassword and settings.showPermaLogin from FlexForm will not be merged into extension settings, if the individual value is empty:

plugin.tx_felogin_login.ignoreFlexFormSettingsIfEmpty = showForgotPassword,showPermaLogin

If an extension already defined ignoreFlexFormSettingsIfEmpty, integrators are advised to use addToList or removeFromList to modify existing settings as shown in the following example:

plugin.tx_felogin_login.ignoreFlexFormSettingsIfEmpty := removeFromList(showForgotPassword)
plugin.tx_felogin_login.ignoreFlexFormSettingsIfEmpty := addToList(domains)

It is possible to define the ignoreFlexFormSettingsIfEmpty configuration globally for an extension using the plugin.tx_extension TypoScript configuration or for an individual plugin using the plugin.tx_extension_plugin TypoScript configuration.

Extension authors can use the new PSR-14 event \TYPO3\CMS\Extbase\Event\Configuration\BeforeFlexFormConfigurationOverrideEvent to implement a FlexForm override process in a custom extension based on the original FlexForm configuration and the framework configuration.

Additionally, the new Extbase TypoScript configuration is used in EXT:felogin to ensure that empty FlexForm settings are not merged into extension settings.

MyVendor\MyExtension\FlexForm\EventListener\MyEventListener:
  tags:
    - name: event.listener
      identifier: 'my-extension/custom-absolute-path'

<?php

declare(strict_types=1);

namespace MyVendor\MyExtension\FlexForm\EventListener;

use TYPO3\CMS\Extbase\Event\Configuration\BeforeFlexFormConfigurationOverrideEvent;

final class MyEventListener
{
    public function __invoke(BeforeFlexFormConfigurationOverrideEvent $event): void
    {
        // Configuration from TypoScript
        $frameworkConfiguration = $event->getFrameworkConfiguration();

        // Configuration from FlexForm
        $originalFlexFormConfiguration = $event->getOriginalFlexFormConfiguration();

        // Currently merged configuration
        $flexFormConfiguration = $event->getFlexFormConfiguration();

        // Implement custom logic
        $flexFormConfiguration['settings']['foo'] = 'set from event listener';
        $event->setFlexFormConfiguration($flexFormConfiguration);
    }
}

Impact

Empty FlexForm extension settings can now conditionally be excluded from the FlexForm configuration merge process.

Also, it is now possible again to use global TypoScript extension settings in EXT:felogin, which previously might have been overridden by empty FlexForm settings.

In addition, with the new BeforeFlexFormConfigurationOverrideEvent it is now possible to further manipulate the merged configuration after standard override logic is applied.