Black bar on right side of screen on iPhone

I’m using the free version of Colormag for my blog, and a few days ago I noticed a black bar running all the way down the right side of the screen when viewing my blog posts on an iPhone.

After some trial and error I found the problem: a textarea next to the right of the Comment box. On small screens, this textarea pushes the page left, thus displaying the page’s black background on the right.

I’ve solved the problem by hiding this textarea with styling:

p.comment-form-comment textarea[name=”comment”] {
display:none!important;
}