I've checked your website. It appears your Editor outputs code differently.
While in Markdown Editor and other Editors the code is outputted like this:
Yours is just a raw <pre> without a <code> tag as a child element. To fix this problem just give the ability to scroll to the <pre> tag instead of <code> by default.
Open muffin theme folder, go to CSS > custom-styles.css and paste the following bit of code and save it.
If you can't see the changes after completing the steps, open the file qa-theme.php
On line 54 - At the end of the link, change this link part.
Adding this little version formula at the end, will notify your browser that there's a new version of this file and will force to clear website cache and update the modifications for everybody that previously visited your site.