Thank you for the credentials, after quite some time of investigating this I still don't get why your sidebar text widget is defaulting to visual editor, resulting to wrap its content in a unnecessary
<p> tag. This
<p> tag is the one that causing those white line issue.
If we can prevent the Wordpress from generating those unnecessary
<p> tags, we might resolve this issue. To do this, please setup a new child theme for X. Then add this on your child theme's functions.php file
remove_filter( 'the_content', 'wpautop' );
remove_filter( 'the_excerpt', 'wpautop' );
Don't forget to activate your child theme.
Let us know how it goes,