TYPO3 Core

Another solution is to add a new feature to the type="select" named "exclusiveKeys". Here you can list (for classic multiple selector boxes) a comma delimited list of keys that exclude all other keys when set.

I think it makes only sense if maxitems>1 and renderMode is not set.

To add some JS code that does the trick on the client side this form rendering function is affected:

class.t3lib_tceforms.php:getSingleField_typeSelect_multiple()

To enforce this on serevr side I think here are some changes needed:

class.t3lib_tcemain.php:checkValue_group_select

Shouldn't be too hard to do and is perhaps reusable for other parts of the Core or some extensions.

The attachment "exclusievKeys.patch" implements the suggested changes to the TCA, the form rendering and the form submission handling.

I've uploaded a new version with cosmetical changes and another oen by Michael Stucki that applies the new TCA features to the access fields of "pages" and "tt_content".

Fixed in CVS.

IMPORTANT: Somehow I dorked the commit. The bug is not fixed in 4.0! Please use 4.0.1 or later, if it is an issue for you.

Ah, now I understand! I noticed this two days ago and wanted to investigate it later... Thanks for taking care of it!