DynamicPreferenceCombiner returns early causing subsequent views in ForEach to not render

[Kyle-Ye]

Description

When a view that does not return a display list (such as Spacer) is placed in a dynamic container like ForEach, any subsequent views fail to render because their display lists are never combined.

Reproduction

struct ContentView: View {
    var body: some View {
        VStack {
            Color.red
            Color.green
            ForEach(0 ..< 1) { index in
                Spacer()
                Color.blue
            }
        }
    }
}

In this example, Color.blue fails to render because Spacer before it does not return a display list, causing DynamicPreferenceCombiner to return early.

Root Cause

An item in a dynamic container may be missing a preference if it does not return a display list:

struct MyEmptyView: View, PrimitiveView, UnaryView {
    func _makeView(view: _GraphValue<Self>, inputs: _ViewInputs) -> _ViewOutputs {
        _ViewOutputs()
    }
}

Spacer is an example of such a view. When placed in a dynamic list, the preference combining logic returns early, preventing subsequent views from being processed.

Related

• PR Fix DynamicPreferenceCombiner returning early #659 (partial fix that doesn't cover all cases)