The Select Next Layer command selects the
layer just underneath the active layer in the layer stack. The command
highlights the layer in the Layers Dialog and makes it the new active
layer. If the active layer is already at the bottom of the stack, this
menu entry is insensitive and grayed out.
Note that on a standard Windows-style English keyboard, the
default shortcut Page_Down does
not refer to the key on the numeric keypad, but to the other
Page_Down key in the group of six keys to the
left of the numeric keypad.
Activating the Command
You can access this command from the image menubar through
Layer->Stack->Select Next Layer,
or by using the keyboard shortcut Page_Down,
or the “Down-arrow” key,
or simply by clicking on the layer name in Layers Dialog.