fix: prevent building component overlay with stale layout

[nicksenger]

In the implementation of on_event for a component's overlay (both pure and impure), after processing local messages which may mutate the state and in turn alter the output of Component::view, the cached element and overlay must be rebuilt. Currently, it is rebuilt with a layout tree which is potentially stale, because the shape of the layout tree is dependent upon the cached element which may have changed.

I've added a preliminary fix here, which is to rebuild the cached element only after processing any local messages, then attempt to rebuild the overlay at the beginning of on_event if it is absent.

I'll open this as a draft because it feels a bit odd to intentionally leave the cached overlay element as None after on_event. I haven't yet thought of another way to resolve this though.

[hecrj]

Good catch!

I think the root cause of the issue here is in the UserInterface::update implementation. Specifically, we are not relayouting the base layer when an overlay invalidates the layout through a Shell, causing the layout to be stale for further events.

I have tried to fix this in 296e157. I needed to rewrite the code to use a for loop instead of iterators as well as use ManuallyDrop to please the borrow checker, but it seems to be happy in the end.

Could you test if these changes fix the issue?

[nicksenger]

That explanation makes sense, and the solution looks good!

I had to make one small change in order for it to fix the issue though:

Since the component overlay will always invalidate the layout when there are local messages produced in on_event, and invalidation of the layout now results in the overlay being rebuilt in UserInterface::update, the overlay no longer needs to be rebuilt after processing local messages in the component's on_event (which is what triggered the panic, since it was still using the same layout to produce the overlay from the newly rebuilt cached element).

[hecrj]

Makes sense. Less code to worry about!

Let's merge 🚢