Can't interact with widgets added to layers other than Background

just change the egui_template code into:

```rs
ui.with_layer_id(egui::LayerId::new(egui::Order::Middle, egui::Id::new("mid")), |ui| {
ui.add(egui::Slider::new(value, 0.0..=10.0).text("value"));
if ui.button("Increment").clicked() {
*value += 1.0;
}
});
```

and the widgets couldn't be interacted with any more. I suppose interactions are allowed in other layers because `egui::LayerId::allow_interaction` returns true? No more info could be found in the doc. It seems to be a bug.

Start by reproducing the provided egui_template example with a widget in a Middle layer, then inspect LayerId::allow_interaction and the surrounding input-handling code. Done means the slider and Increment button remain interactive outside the Background layer, with a regression test or example demonstrating the behavior.

Written by the indexing model from the issue text.