If select category won't work on FreeFlarum, you can try this.
.Modal-content{ position: absolute; top: -310 px; left: 100 px; z-index: 1 ;}
Change the CSS name of your category popup, you can find how to do that here, how far on the top it needs to be, how far left, and make the z-index higher than the comment form behind it to make it appear on top.
You may need to put the z-index beside a different CSS name. It's all about finding what works for you.
Editing this may make your Login, Sign up and Forgot password popups invisible, if this is the case, change their positions just as you have done here, you may need to delete what you've wrote in the CSS box so the login links appear again, edit their position via CSS, and re paste this CSS.
