Show WYSIWYG button below custom category attribute

Question

I added category attribute (textarea) and Magento stops displaying WYSIWYG button if I use custom renderer for that textarea field. I want to ask I anyone knows why this happens?

Here are my steps. I added category attribute, a simple textarea with WYSIWYG button:

$installer->addAttribute('catalog_category', 'my_attr1', array(
    'label'             => 'My attribute 1',
    'type'              => 'text',
    'input'             => 'textarea',
    'visible_on_front'  => true,
    'wysiwyg_enabled'   => true,
    'is_html_allowed_on_front' => true,
    'visible'           => true,
    'required'          => false,
    'backend'           => '',
    'frontend'          => '',
    'searchable'        => false,
    'filterable'        => false,
    'comparable'        => false,
    'global'            => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_STORE,
));

All works correct, WYSIWYG button is displayed below the field.

Then I added second attribute my_attr2 to make some tests. That second attribute use my custom renderer class (see 'input_renderer' => 'my_custom') because I need to extend that field with additional functionality. I checked that attribute was added correct and works 100% correct. That class My_Custom extends Magento's default textarea renderer Varien_Data_Form_Element_Textarea so it looks and works exactly the same as standard textarea field.

But for some reason Magento stops displaying WYSIWYG button if I use custom renderer.

$installer->addAttribute('catalog_category', 'my_attr2', array(
    'label'             => 'My attribute 2',
    'type'              => 'text',
    'input'             => 'textarea',
    'input_renderer'    => 'my_custom', //<---custom
    'visible_on_front'  => true,
    'wysiwyg_enabled'   => true,
    'is_html_allowed_on_front' => true,
    'visible'           => true,
    'required'          => false,
    'backend'           => '',
    'frontend'          => '',
    'searchable'        => false,
    'filterable'        => false,
    'comparable'        => false,
    'global'            => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_STORE,
));

Here's the rendered class (located in file lib/My/Custom.php). I checked that Magento for sure use this class when rendering the second attribute (I added logging to the constructor to make sure it works).

<?php
class My_Custom extends Varien_Data_Form_Element_Textarea
{
    public function __construct($attributes=array())
    {
        parent::__construct($attributes);
        Mage::log("My_Custom is working");
    }
}

Even If I remove all the code from the renderer class lib/My/Custom.php to make sure I didn't break or override anything, the WYSIWYG button is still not displayed below the second attribute:

<?php
class My_Custom extends Varien_Data_Form_Element_Textarea
{
}

And as soon as I update my_attr2 and remove the custom renderer class, then the WYSIWYG button is displayed correct below the textarea (just like it should be).