If [code]true[/code], the window background is transparent and window frame is removed.
Use [code]get_tree().get_root().set_transparent_background(true)[/code] to disable main viewport background rendering.
[b]Note:[/b] This property has no effect if [b]Project > Project Settings > Display > Window > Per-pixel transparency > Allowed[/b] setting is disabled.
[b]Note:[/b] This property is implemented on Linux, macOS and Windows.
[b]Note:[/b] This property is implemented on HTML5, Linux, macOS and Windows.