The added widget on Customizer Widget menu is not hidden from available list.
- You can use is_customize_preview() instead of is_admin().
- You may use the same DOM structures and names even if the shown position is opposite. This is the comment from Customizer's view.
<!-- For Widget Customizer, many widgets try to look for instances under div#widgets-right, so we have to add that ID to a container div in the Customizer for compat -->
But the rendering and DOM generation timing are different from Administration Screen, jQuery.find() could not find the element.
The added widget on Customizer Widget menu is not hidden from available list.
<!-- For Widget Customizer, many widgets try to look for instances under div#widgets-right, so we have to add that ID to a container div in the Customizer for compat -->But the rendering and DOM generation timing are different from Administration Screen, jQuery.find() could not find the element.