How do I turn off the Junk Yard curtain?
Short answer: Switch off Open a strip of scraps when the pointer hits the screen edge in Settings → Junk Yard. The board still works from its tab and shortcut.
In Settings → Junk Yard, switch off Open a strip of scraps when the pointer hits the screen edge. The rest of the Junk Yard keeps working — you can still open the board from its tab or with ⌃⇧J.
If it's only appearing too easily, you may prefer a longer Open delay instead. See The curtain opens when I reach for the menu bar.