How do I stop a two-finger tap from deleting scraps in the curtain?
Short answer: Switch off Two-finger tap deletes a scrap in Settings → Junk Yard (Edge curtain section). You can still delete with a tile's trash button.
In Settings → Junk Yard, under Edge curtain, switch off Two-finger tap deletes a scrap. Tiles then can only be removed with their hover trash button or from the board.
It's worth doing if you often right-click by accident on a trackpad, because a deleted scrap can't be brought back. Pinned scraps are protected from this gesture either way.