Section(header: Text("Tip amount")) {
                // right now we are in a trailing closure
                    Picker("Tip percentage",  selection: $tipPercentage) {
                    // and now we are inside a second closure
                        ForEach(0 ..< tipPercentages.count) { // here, tipPercentages doesn't need to be prefixed by 'self.'
                        // and now we are inside a third closure
                            Text("\(self.tipPercentages[$0])%") // here, tipPercentages *does* need to be prefixed
                        }
                    }.pickerStyle(SegmentedPickerStyle())
                }

Why do we need to prefix tipPercentages with self. inside the third closure, but not in the second? This has to be a language rule, but I'm just not getting it.

I do not think the self is necessary in this case