[NeverMorewd]

Summary

Replaces #56, which had to be closed because its branch (codex/region-floating) predates PR #55's squash-merge and compared against main as 15 conflicting commits. This PR carries the exact same single commit, cherry-picked onto a fresh branch off current main - clean, no conflicts, 1 commit.

PR #55 was merged before this commit finished pushing, so the fix below never landed on main.

• ViewPlacementCoordinator: the contended-acquisition path (EnterSlowAsync) set its reentrancy marker via an AsyncLocal<int> write made inside an async method — which never becomes visible to that method's own caller once it returns (the compiler-generated state machine restores the caller's captured ExecutionContext on resume, even when the method never truly suspends). This reproduced both the original self-deadlock the reentrant-lease fix was meant to prevent, and a spurious InvalidOperationException from RestorePlacementAsync's IsHeld() guard, whenever the first lease acquisition for a region happened to be genuinely contended. Replaced the AsyncLocal<int> depth counter with an AsyncLocal<Guid> flow id established once per logical operation via a new EnsureFlow(), called synchronously at every entry point before EnterAsync — since AsyncLocal always propagates correctly parent→child, only the old child→caller-after-suspension direction was broken. The "who holds this region" marker itself is a plain field on the shared per-region gate object, not an AsyncLocal, so writing it from inside the async slow path is fine.
• WeakUnloadObserver: synchronized the ConditionalWeakTable read-modify-write sequence (Add throws on a concurrent duplicate-key race).
• RegionNavigationService: set up the indicator host in RequestNavigateAsync before job scheduling and lease acquisition — not just inside CreateNavigateTask — so an error indicator can still be shown if either of those throw before reaching it.

Added ViewPlacementCoordinatorTests covering both the fast-path reentrant case and the previously-broken contended-then-reentrant case.

Test plan

• dotnet build tests/AsyncNavigation.Tests (Release) — 0 errors
• dotnet test tests/AsyncNavigation.Tests — 157 passed
• dotnet test tests/AsyncNavigation.E2E.Tests — 9 passed
• New ViewPlacementCoordinatorTests (2 tests) reproduce and verify the fix for the contended-lease self-deadlock/IsHeld gap
• This exact commit already verified green on all 3 OS via manual CI dispatch (see fix: close reentrancy gap on the contended placement-lease path #56)

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com