This is a site/theme specific issue, you can fix it using this css code:

#wpforo #wpforo-wrap .wpf-post-create .wpf-subject {
    width: 98%;
}

Oh.. Thank you for your reply!

But I think that it's not the problem of my site & theme.

I can also see the same layout problem in this site(wpforo.com) whenever I use IE.

Ok, I forgot that you're talking about IE11, I'll try to find IE11 somewhere and test it.