I'd like to say that I am moderately experienced when it comes ot implementing and troubleshooting compatibility errors with mods, but this is something I haven't seen before. Upon trying to load in a series of 61 mods using a joint vers. of LiteLoader and Forge 1.12.2, I found that the game is duplicating itself as a seperate window within the game window itself. Its hard to explain, so I've taken the liberty of sharing a screenshot
I believe the conflict has something to do either with LiteLoader, or a particular mod I'm using with LiteLoader (the only ones of which being Mine Little Ponies and HDSkins), though I will post a complete mod list ot help regardless. If there is any information I am forgetting to post please let me know, any help would be appreciated!
EDIT: I forgot to mention, you can resize the Minecraft window inside the game window independently, unsure if relevant but figured it's important enough to note. Also, I am using Forge1.12.2-22.214.171.12438.