TYPO3 Core

I have the same issue. Somehow it only seems to be an issue of you use type array. I will see what I find out.

Did any of the watchers find a solution to this yet?

I'm experiencing this problem after updating from 7.6 as well.

If someone needs examples, this public extension of ours has the problem.
https://github.com/AOEpeople/TYPO3-Google-Tag-Manager

I'm not quite there yet, but to me, it looks like

$GLOBALS['TCA']['tt_content']['types']['list']['subtypes_addlist'][$pluginSignature] = 'pi_flexform';

If I fake it and add it manually to

$GLOBALS["TCA"]["tt_content"]["types"]["list"]["showitem"] = $GLOBALS["TCA"]["tt_content"]["types"]["list"]["showitem"] . ',pi_flexform';

it works again. But I don't like the solution. The bug is still there.

Here is the change that removed the usages of

$GLOBALS['TCA']['tt_content']['types']['list']['subtypes_addlist']

https://review.typo3.org/c/Packages/TYPO3.CMS/+/41933/101/typo3/sysext/backend/Classes/Form/Container/FullRecordContainer.php#b59

Steps to reproduce the problem, I have choose TYPO3 8.7 for the steps as the extension I have the problem with is compatible with that.
The problem has been confirmed in newer TYPO3 version like 9.5 and 10.4 as well by me, but I don't have public extension to reference for that.

1. Start a TYPO3 Installaction (e.g. with DDEV) TYPO3 Version 8.7
2. Install https://github.com/AOEpeople/TYPO3-Google-Tag-Manager
3. Add following PageTS to a PageTS

# add GTM plugin to the content wizard
mod.wizards.newContentElement.wizardItems.plugins.elements.googletagmanager_datalayer {
    iconIdentifier = tx-google-tag-manager
    title = Plugins : Google Tag Manager
    description = Google Tag Manager Data Layer
    tt_content_defValues {
        CType = list
        list_type = googletagmanager_datalayer
    }
}

mod.wizards.newContentElement.wizardItems.plugins.show := addToList( googletagmanager_datalayer )

4. On the same page, add a new elment of the "Google Tag Manager", It's important that you select the "google tag manager" directly, so that the element will not be saved first.

5. Go to the plugin tab, and click "Add Datalayer", and you will see that and "foobar" element is added.

A work around is to save the element before clicking the "Add Datalayer" and after that the element will look correct when added.

I have created a V10 branch of the extension.
https://github.com/AOEpeople/TYPO3-Google-Tag-Manager/tree/tnm/v10test

It Can be that the extension isn't compatible with 10 LTS, but changing the composer.json and ext_emconf.php is enough to reproduce the problem in 10.4 as well.

it works with the following flexform

<settingsdatalayervars>
                        <title>DataLayer Values:</title>
                        <type>array</type>
                        <section>1</section>
                        <el>
                            <datalayervar>
                                <type>array</type>
                                <title>DataLayer Value</title>
                                <el type="array">
                                    <label>
                                        <TCEforms>
                                            <label>Label:</label>
                                            <config>
                                                <type>input</type>
                                                <eval>trim</eval>
                                            </config>
                                        </TCEforms>
                                    </label>
                                    <value>
                                        <TCEforms>
                                            <label>Value:</label>
                                            <config>
                                                <type>input</type>
                                                <eval>trim</eval>
                                            </config>
                                        </TCEforms>
                                    </value>
                                </el>
                            </datalayervar>
                        </el>
                    </settingsdatalayervars>

it currently does not work with having settings.datalayersvars. so I guess it is again a missing escape thing

Thanks for your input Georg. It doesn't change anything in my setup.

Still failing the same way as before.

I tested with 10.4

I have made a small screencast with to show the problem
https://www.dropbox.com/s/a045mmnnm6k1m5h/flexform.mp4?dl=0

I think I have located the Problem, but not the solution.

The $formDataCompilerInput['databaseRow'] is besides 'uid' empty, therefor there will never be found any default values for the list_type.

FormFlexAjaxController.php Line: 74

      $formDataCompilerInput = [
            'tableName' => $tableName,
            'vanillaUid' => (int)$vanillaUid,
            'command' => $command,
            'recordTypeValue' => $recordTypeValue,
            'processedTca' => $processedTca,
            'flexSectionContainerPreparation' => $flexSectionContainerPreparation,
        ];
        // A new container on a new record needs the 'NEW123' uid here, see comment
        // in DatabaseUniqueUidNewRow for more information on that.
        // @todo: Resolve, maybe with a redefinition of vanillaUid to transport the information more clean through this var?
        // @see issue #80100 for a series of changes in this area
        if ($command === 'new') {
            $formDataCompilerInput['databaseRow']['uid'] = $databaseRowUid;
        }
        $formData = $formDataCompiler->compile($formDataCompilerInput);

Therefore the flexform isn't added to the showItems

Adding this to the FlexFormAjaxController, solves the problem for me. I'll sleep on this over the weekend, and see if I find any problems or better approaches before I push it for review.

 if ($command === 'new') {
     $formDataCompilerInput['databaseRow']['uid'] = $databaseRowUid;
+    $dataStructureKey = $queryParameters['dataStructureIdentifier']['dataStructureKey'];
+    $subtypeValue = substr($dataStructureKey, 0, strpos($dataStructureKey, ','));
+    $formDataCompilerInput['databaseRow'][$processedTca['types'][$recordTypeValue]['subtype_value_field']] = $subtypeValue;
 }

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/68152

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/68152

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/68152

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/68152

Patch set 6 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/68152

Patch set 1 for branch 10.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/72147

Patch set 2 for branch 10.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/72147