How do I make the curtain slide out from the screen edge?
Short answer: Move your pointer to the docked screen edge (top or right, set under Dock to in Settings → Junk Yard) and pause briefly — the strip slides in.
Move your pointer all the way to the screen edge the curtain is docked to — the top or the right — and pause for a moment. After a short delay the strip slides into view.
Check which edge it's set to under Dock to in Settings → Junk Yard. The curtain needs Open a strip of scraps when the pointer hits the screen edge switched on there, and the Junk Yard itself enabled.