Styx: Master of Shadows on the PC just ignores the mouse wheel which is required in order to be able to cycle through items.
To fix the issue, edit
My Documents\My Games\Styx\StyxGame\Config\StyxGame.ini and append the following lines at the bottom of the file:
Bindings=(Name="PageUp", Command="InputItem_Left | OnRelease InputItem_Left_Release") Bindings=(Name="PageDown", Command="InputItem_Right | OnRelease InputItem_Right_Release")
The lines will map the Page Up and Page Down to mouse wheel up and mouse wheel down respectively.
PageDown can be changed to the name of any other desired key.