Key Strategy Trade‑off for LazyColumn with Paging 3 Placeholders

Direct Answer

Use a hybrid key that combines the domain ID with a monotonically increasing generation token:

val pagingState = remember { mutableStateOf(pager.flow.collectAsStateWithLifecycle().value) }

LazyColumn(
    state = rememberLazyListState(),
    key = { index, item ->
        item?.let { "${it.id}#${pagingState.value?.generation ?: 0}" } 
            ?: "placeholder#$index#${pagingState.value?.generation ?: 0}"
    }
) { items(...) }

This guarantees uniqueness for real items and placeholders, survives config changes, and avoids lambda mutation because the generation token is derived from immutable PagingState.

Why Other Strategies Fail

Domain ID only (key = it.id)

Fails because Paging 3 inserts null placeholders by default. A null-safe fallback like it?.id ?: -position creates collisions when RemoteMediator upserts reuse the same server ID across refreshes—Compose sees duplicate keys and throws.

Positional keys (key = index)

Indices shift when preceding pages insert new items. After a config change or process death, LazyListState restores firstVisibleItemIndex, but that index now maps to a different logical item, causing scroll jumps.

Disabling placeholders

PagingConfig(enablePlaceholders = false) eliminates nulls and lets you use key = it.id, but the initial render shows gaps until the first page loads. Scroll restoration still works because LoadState.NotLoading emits only after data arrives, but users see flicker on slow networks. A skeleton composable driven by LoadState.Loading is a better UX than placeholders.

Implementation Steps

1. Expose a generation token from RemoteMediator. Increment a counter in refresh() or use PagingState.lastRefreshKey as the token.
2. Collect PagingState in a stable holder. val pagingState = remember { mutableStateOf(pager.flow.collectAsStateWithLifecycle().value) } ensures the lambda captures a stable reference.
3. Compose the key lambda without capturing mutable locals. The lambda above reads only pagingState.value.generation and the item—both stable across recompositions.
4. Verify with Layout Inspector. Confirm each node has a unique key attribute matching the pattern id#gen or placeholder#index#gen.

Verification Checklist

• Rotate device: scroll position should restore to the same logical item.
• Trigger RemoteMediator.refresh(): no duplicate-key crashes, placeholders get fresh keys.
• Inspect LazyListState.scrollPosition before/after config change via onGloballyPositioned logging.

One Missing Detail

Does your RemoteMediator implementation already expose a generation/load ID (e.g., via a refreshGeneration property), or do you need to derive one from PagingState.lastRefreshKey or a side-effect counter? The answer determines whether step 1 is a one-liner or requires a small wrapper.