The window.transprency might affect correct rendering of some widgets, use it wisely.
Window.transparency read/write property
Get or set the window's transparency intensity, a number value from 0 (full transparency) to 255 (full opacity, the default value).
Example
local ui = require "ui"
local win = ui.Window("Window.transparency example", 420, 300)
win:center()
local label = ui.Label(win, "")
label.fontsize = 40
label.style = { bold = true }
async(function()
while win.transparency > 0 do
label.text = math.floor(win.transparency/255*100).."%"
label:center()
win.transparency = win.transparency-1
sleep(10)
end
win:hide()
end)
ui.run(win):wait()