AbstractContainerScreenMixin.java

@Mixin(AbstractContainerScreen.class)
public abstract class AbstractContainerScreenMixin implements ScreenProcessorProvider {
    @Shadow @Nullable protected Slot hoveredSlot;

    @Shadow protected abstract void slotClicked(Slot slot, int slotId, int button, ClickType actionType);

    @Unique private final ScreenProcessor<?> screenProcessor = new AbstractContainerScreenProcessor<>((AbstractContainerScreen<?>) (Object) this, () -> hoveredSlot, this::slotClicked);

    @Override
    public ScreenProcessor<?> screenProcessor() {
        return screenProcessor;
    }
}

CreativeModeInventoryScreenMixin.java

@Mixin(CreativeModeInventoryScreen.class)
public abstract class CreativeModeInventoryScreenMixin extends AbstractContainerScreenMixin {
    @Unique private final CreativeModeInventoryScreenProcessor controlify$screenProcessor
            = new CreativeModeInventoryScreenProcessor((CreativeModeInventoryScreen) (Object) this, () -> hoveredSlot, this::slotClicked);

    @Override
    public ScreenProcessor<?> screenProcessor() {
        return controlify$screenProcessor;
    }
}