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First of all, it is a somehow a bug in the module, because in the plugin class xoopsformelement is tested, while the XoopsFormDhtmlTextArea class is used.
The included xoopsformloader.php includes all xoopsformxxxx classes, including the two before mentioned.
So, the author of the module assumed that these classes will be always loaded togheter and not individually. I don't know if that was a rightful assumption and that it was upholded always in previous versions of the XOOPS core. (Probably yes, because nobody complained before)
But fixing the modules Smarty plugin, will always work in every XOOPS version.
Technically, for PHP there is no reason, why all these classes should be included at once and you can use most individually or with only a few others having some benefit in time and memory.
Question is now if there was some API rule that the form elements needed to be included always togheter and not to be loaded individually. If so, this could be considered as a compatibility bug.
Note that, if the classes are not loaded by the core, you could run into the same problem, when you have another module that does also only the include of some individual formelements.
