@NamelessCoder What I just think: can we put the call to CompatibilityRegistry for the DEFAULT_SHOWITEM into an if condition with TYPO3_MODE === 'BE' ?

I have made an update which now checks for TYPO3_MODE === 'BE'

Please wait: I'll better remove the TCA data from ext_localconf.php to reduce the memory consumption. It's better to use a compatibility class for the TCA data with CompatibilityRegistry.

@NamelessCoder maybe we can combine the same TCA configuration in a separate class and can use it for fluidcontent and flux. It's redundant data.
What do you think?
see FluidTYPO3/fluidcontent#387

I changed to a public static property for $defaultShowItem so we can use it also in fluidcontent for TCA in Configuration/TCA/Overrides/tt_content.php

Or should we use \FluidTYPO3\Flux\Helper\ContentTypeBuilder::addBoilerplateTableConfiguration to create the TCA in fluidcontent?
see also FluidTYPO3/fluidcontent#388

If possible use addBoilerplateTableConfiguration - also if that means we nedd to add an argument for example.

@NamelessCoder Hi Claus.
the PR is now ready, but take a look at this
https://github.com/FluidTYPO3/flux/blob/development/Classes/Helper/ContentTypeBuilder.php#L204

$GLOBALS['TCA']['tt_content']['columns']['pi_flexform']['ds']['*,' . $contentType] = [];

had no effect all the time, because it should be

$GLOBALS['TCA']['tt_content']['columns']['pi_flexform']['config']['ds']['*,' . $contentType] = [];

but then the empty array is wrong

I have removed it and now $GLOBALS['TCA']['tt_content']['columns']['pi_flexform']['config']['ds']['default'] is used for default dataStructure.

'default' => '
	<T3DataStructure>
	  <ROOT>
		<type>array</type>
		<el>
			<!-- Repeat an element like "xmlTitle" beneath for as many elements you like. Remember to name them uniquely  -->
		  <xmlTitle>
			<TCEforms>
				<label>The Title:</label>
				<config>
					<type>input</type>
					<size>48</size>
				</config>
			</TCEforms>
		  </xmlTitle>
		</el>
	  </ROOT>
	</T3DataStructure>
'

You can skip the new class with static properties containing TCA (generally speaking, this pattern is not desired; metadata should not be configured in a class). Instead, use the CompatibilityRegistry in the TCA override file for tt_content and apply the TCA there as well. The result then gets cached which is fine in this case (better than not caching, really).

Now the CompatibilityRegistry is used in TCA override file for tt_content.