Leading up to the release of WordPress 4.8 there was no lack of feedback on how the editor and associated widget formatting potentially screwed up content. The formatting errors effected everything - including blocks like AdSense code. To disable these formatting features, use the following filters.
Copy and paste the WordPress function into your theme's
functions.php file or, if you sensibly have one installed, your custom functions plugin.
Note that these filters will disable the widget feature globally. If using text in the widget, avoid switching over to the visual editor because the text will inherit the wrath of WP's formatting over again.
A HTML widget is proposed for version 4.8.1.