Casio fx4000p repair question

[Tugdual]

I just completely serviced a Casio fx-4000p and after the reassembly, it didn’t switch on. I found on some pictures that there is a small white dot near the on switch which seems to pressure the switch film against the board. I added a piece of tape there and indeed the calculator operates well now. Can sbdy give me more detail about that white thing? What material is it, how does it stick to the film? I haven’t seen it while opening the calculator.