Menus

commands.add("kit", function(player)
    local m = player.ui.menu{
        title = "Trains",
        items = {
            { item = "coal", name = "Steam locomotive", lore = { "right click a rail holding it" } },
            { item = "redstone", name = "High-speed electric" },
            "iron_ingot",                 -- a bare name works, as everywhere
        },
        on_pick = function(p, n)          -- n is the item picked, 1 up
            p:give(({ "coal", "redstone", "iron_ingot" })[n])
        end,
        on_close = function(p) end,       -- put away without picking
    }
    -- m:destroy() takes it down again; m:update(items) redraws it in place
end)

The rules:

• Every client draws a menu. Java (1.7 to 26.3) shows a chest. Bedrock shows a form with one button per item, labelled with name or the item’s name. No player:supports check is needed.
• An item is an item (see “items”) with name, lore, color, dye, glint, skin and count. The count shows on the stack, for a price or a quantity.
• on_pick may task.wait.
  • On Java the menu stays open. Destroy it in on_pick if one pick is enough.
  • A Bedrock form closes on the press. Open it again for another pick.
• menu:destroy() and a Bedrock pick do not fire on_close.
• menu:update returns false after the player closes the menu. Picks still answer by position. A Bedrock form keeps its first items.
• A new menu replaces the old one.
• Only the pack that opened a menu gets its picks. The menu ends when the script ends, the player leaves, or the pack calls destroy.
• Under a Java menu, the player’s inventory shows. Clicks in it change nothing.