TYPO3 Core

The patch provided by Andreas Allacher breaks flexforms type "select" renderType multipleSideBySide - the icons for ordering and removing won't work any more.

Did you copy both files or only FormEngineValidation.js
If used in 7.6 have you tried to apply the patch as patchset (especially FormEngineValidation.js) instead of copying the files - there might have been further changes in master.

Yes I copied both files.
No not tried as patchset - it was working when using files from Typo3-8 branch

The commit fixes the behaviour because it adds has-error css class the to the field itself everytime the value
markParent was set to true.

Which was not the case prior.

For instance if you have a TCA configuration like:

<?php
$GLOBALS['TCA']['test_table']['columns']['test_column'] = [
    'config' => [
        'type' => 'select',
        'renderType' => 'selectMultipleSideBySide',
        'minitems' => '1,
        'items' => [
            ['foo', 'foo'],
            ['bar', 'bar']
        ]
    ]
]

And you haven't selected an item, you will be able to store the data and the tab will have an error mark but not the field itself, the reason:
https://git.typo3.org/Packages/TYPO3.CMS.git/blob/refs/heads/TYPO3_7-6:/typo3/sysext/backend/Resources/Public/JavaScript/FormEngineValidation.js#l296

Look at line 307 there. In comparison to 314.

That is not the only place where the corresponding class is not appended, e.g. type group and inline don't add the class at all.

I re-checked there are also some instances where the validation might not work in master either, e.g. SelectSingleElement maxitems = 1 also excepts a selected value that is empty (empty string), e.g. if one wants to create an option like "Please choose".
Although that might be the correct behaviour as one just has to add 'eval' => 'required' to achieve that check but only if one uses strings (as zero would not be an allowed value with required).

Of course if there is not please choose option there always will be a selected value but it might accidentally be the default value instead of the wanted value but that might be a feature request instead.

Just further information: using range and lower would also support disallowing 0 (as long as there are no values lower). So having minitems with a single select can be achieved by using other evals.

But that in 7.6 no error is displayed / checked for certain fields, see comment #5 ( https://forge.typo3.org/issues/75803#note-5 )
for details has to be fixed

Patch set 2 for branch TYPO3_7-6 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/48336

It's fixed in Typo3 7.6.10 - I tested yesterday.