Working Curtains in Vanilla Minecraft! Hope you enjoy!
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Commands Used:

1. /gamemode 3 <playername> | For no hotbar shown and invisibility from other players.

2. /tp <playername> ~ ~ ~ | Enter the coordinates in the "~" symbols that are shown when you step outside the window.

3. (Make sure to put 4 repeaters down before this command, on full power) /gamemode 1 <playername> | To turn back to normal.

4. /tp <playername> ~ ~ ~ | Enter the coordinates in the "~" symbols that are shown when you step next to the curtain (Inside).

All Done!


-Evenger