wizard_edit doesn't work in flexforms
The popup closes immediately because of a failing TCA check.
This doesn't work in 7.6. The code in 8.4 seems to be the same. See also Bug #20420
TYPO3\CMS\Backend\Controller\Wizard\EditController::main() do some checks using TCA and some GET parameters. All checks are done using TCA configuration:
$config = $GLOBALS['TCA'][$table]['columns'][$field]['config'];
Unfortunately this will not work for flexforms because we need here the configuration of the flexform field, not the parent tt_content field (for example).
Attached is a patch. I see that as a proof of concept as it will not work with NEW records and the flexform handling is a bit un-generic. The comments in the code should that make clear.
Here's a flexform field for testing:
<settings.collections> <TCEforms> <label>LLL:EXT:cms/locallang_ttc.xlf:file_collections</label> <config type="array"> <type>group</type> <internal_type>db</internal_type> <allowed>sys_file_collection</allowed> <maxitems type="integer">999</maxitems> <minitems type="integer">1</minitems> <size type="integer">5</size> <wizards type="array"> <_VERTICAL type="integer">1</_VERTICAL> <edit type="array"> <type>popup</type> <title>Edit</title> <module type="array"> <name>wizard_edit</name> </module> <popup_onlyOpenIfSelected type="integer">1</popup_onlyOpenIfSelected> <icon>actions-open</icon> <JSopenParams>height=350,width=580,status=0,menubar=0,scrollbars=1</JSopenParams> </edit> </wizards> </config> </TCEforms> </settings.collections>