using jQuery.on binding for catching the click in custom-header.js

[DaanMeijer]

I ran into this while building a theme that allows for images in it's custom widgets. The problem:

I want to use wp-admin/js/custom-header.js to bind to my
<a id="choose-from-library-link"> ... </a> ,
but because a widget form is loaded through AJAX, the element does not exist in the DOM yet, when the click is caught.

I propose changing line 18 of wp-admin/js/custom-header.js to
$(document).on('click', '#choose-from-library-link', function( event ) { .

[celloexpressions]

I don't see a problem with making this change per se, but I would not recommend using an admin-specific file that's liable to change and, in this case, possibly even be removed (see #25571), in a theme or plugin. And this change isn't necessary for this file to function as intended for core.

If you want similar functionality to what that file does, copy it into a new, appropriately named file bundled with your theme/plugin and tweak it to fit your needs. Or, look at a tutorial on using the media library in themes and plugins. Also, you shouldn't use an id like that in a widget, as multiple instances of the widget will result in duplicate ids.

[celloexpressions]

Per previous comment, this isn't really needed for core and facilitates invalid use of this file.