It's been a while since I watched Redspark's video, so I'm not sure what he covered. Anyway, for sliding interfaces you need to use Lua script. It's only a few lines so it's nothing complicated.
First things first, you should make sure you have defined the interface area.
Now, inside of the properties tab you need to do 2 things...
1. in the displacement set absolute & now in the offset you should leave x as whatever it currently is & in y you should offset it so that it pushes the interface below the scene.
in my case I'm pushing the interface above the scene because my interface slides in from the top.
2. click on the lightning bolt icon for action on leaving to open up a new window so that you can create an event to happen when the mouse cursor leaves the action area. Inside of this window create an execute a script action part & insert this code into it...
Interfaces["inventory"]:to(1000, {InterfaceOffset = { x = 0, y = 1080 } }, easeQuintInOut)
Don't forget to change the y value in the script to whatever you set in the y offset position in the properties tab.
Next you need a method for sliding in the interface, I'm going to suggest a Lua mouse event handler, though maybe you would prefer to create another interface so that you can add a button that the player can mouse over. Anyway, for the mouse event handler method you will need to go to the script section of the editor & create a new script (make sure you set it as a definition type script in the properties section). Now insert this code into it...
function onMouseEvent(typ, pos)
if pos >= (game.WindowResolution.y - 10) then
if Interfaces["inventory"].Visible and Interfaces["inventory"].InterfaceOffset.y == 1080 then
Interfaces["inventory"]:to(1000, {InterfaceOffset = { x = 0, y = 350 } }, easeQuintInOut)
end
end
end
registerEventHandler("mouseEvent", "onMouseEvent")
It should force the interface to slide out when the mouse cursor is 10 pixels or less from bottom of the screen providing the interface is visible & not already slid out.
Alternatively again, you could create a button on screen or use a keyboard shortcut to toggle the inventory open/closed, or you could use the mouse wheel - but not everyone has a mouse wheel, so that has to be taken into consideration.