I know that your post was made more than a month ago, but I would like to share with you, and anyone else wondering about this issue, my solution. 

Step 1: Download this program:

 https://www.mediafire.com/file/jnsslpz2wl7yjf8/RemoveBorders.rar/file

Don't worry, it's 100% safe, but don't take my word for it, watch this video that explains how the program works:

Step 2: Open the program, then right-click on the icon that should show up in the "hidden icons" tab, and then click on "settings". Check the box that says "auto", then choose a hotkey. I personally use "numpad-minus", but you can use whatever key you want. 

Step 3:  Open TerraCraft. Wait until all of your mods are loaded, then maximize the window. After that, press the hotkey that you assigned. The program should now appear to be full-screen, with no borders.

If you have any questions, feel free to respond to this post. I will do my best to answer them as best I can. 

I hope you found this helpful.