Steps to reproduce:

Create release
On tracklist, click "Add disc" to add a second disc
Choose "Existing tracklist"
Choose album (My album is 梶芽衣子, 銀蝶渡り鳥 (Vinyl 1))
Click "Add disc" to close dialog an fill in track list for disc 2
Collapse disc 1 and disc 2
Click "Add disc" to add a third disc
The dialog box is off the top of the page