It's just a filter, press E. Now I feel kinda dumb, the thought to open the inventory didn't even cross my mind - played too much with a filled out quick bar. That said: Other tutorials (i.e. the "Entity transfer" one) seem to disable clearing the quick bar for tutorial-relevant items, ev...

What did you do? I started the "Rail signals advanced" tutorial. What happened? I accidentally used a middle mouse click on one of the items that have been provided in the quick bar. This has cleared the item, making the tutorial impossible to complete. Even clicking the "reset"...