I was adding six pieces of cover art (three 8 MB jpg files and three ~15 MB jpg files) but they were in the wrong order, so I had to rearrange them. However, the UI bugged out after I had used the up and down arrows on the files a few times. I'm using Firefox 59.0.2 and my memory use went down 1,5 GB after I closed the "add cover art" tab. I had other memory heavy programs running in the background, so I'm guessing that the computer temporarily ran out of memory. Either way, the UI shouldn't bug out like it did.

As you can see in the image, the top jpg file (one of the bigger files) has bugged out while the bottom one still is unchanged, but it will also bug out in the same manner if you use the arrows.