When received in [method Node._input] or [method Node._unhandled_input], returns the mouse's position in the root [Viewport] using the coordinate system of the root [Viewport].
When received in [method Control._gui_input], returns the mouse's position in the [CanvasLayer] that the [Control] is in using the coordinate system of the [CanvasLayer].
When received in [method Node._input] or [method Node._unhandled_input], returns the mouse's position in the [Viewport] this [Node] is in using the coordinate system of this [Viewport].
When received in [method Control._gui_input], returns the mouse's position in the [Control] using the local coordinate system of the [Control].