The Layer to bottom command lowers the active layer to the bottom of the layer stack.
If the active layer is already at the bottom of the stack or if there is only one layer, this menu entry is disabled.
You can access this command from the main menu through Layer → Stack → Layer to Bottom,
Shift キーを押しながレイヤーダイアログの下向き矢印アイコンボタンをクリック
