When attempting to edit a panels page and add content (panelpathnamehere/panel_content), the "add content" cog icon that triggers the options overlay/box/popup/modal is not presented due to an error in javascript that I think is in the Shadowbox Login module.

Safari reports this error as such:

TypeError: 'undefined' is not an object (evaluating 'parent.Shadowbox.isOpen')

Line with error is in login_shadowbox_check.js:
if ( ! is_shadowbox_form && parent.Shadowbox.isOpen() ) {

Which is part of this code in login_shadowbox_check.js:

(function($) {
var is_shadowbox_form;
var current_location;
current_location = document.location.href;
is_shadowbox_form = current_location.indexOf('shadowbox', current_location.length - 'shadowbox'.length) !== -1;
if ( ! is_shadowbox_form && parent.Shadowbox.isOpen() ) {
parent.window.location.href = current_location;
}
}(jQuery));

I have been trying to hunt down the problem module for a very long time now that was messing with things here and there, finally found it! Guess I just didn't want to believe it was Shadowbox (Login) since I wanted that functionality at the time I was initially enabling modules months ago, using colorbox and colorbox-node now.

When disabling the Shadowbox Login module, the error goes away and the Panels Add Content cog icons show up again as they should.