Summary

Follow-up to #225. A self-review of the new race regression suite surfaced two coverage holes — both tests passed, but for the wrong reasons.

1. Replace stress test exercised the wrong race

AnimationInvalidation_ConcurrentWithReplace_DoesNotThrow ordered Interlocked.Exchange after node.Replace(id, next). By the time Exchange ran, Replace had already disposed the old segment, so the ticker's Volatile.Read(ref current) mostly returned a reference to an already-disposed segment. The test was effectively validating "TriggerInvalidation tolerates disposed segment" rather than the advertised live-segment-vs-Replace race.

Fix: Restructure with two tracked elements — a permanently-tracked spinner the ticker fires on (always live), and a separate tracked element that Replace swaps repeatedly. The ticker is always firing on a live subscription, so OnAnimationInvalidated competes with Replace for _contentLock, which is the race the test claims to cover.

2. AfterDispose test passed for the wrong reason

AnimationInvalidation_AfterDispose_DoesNotThrow called node.AppendTracked, then node.Dispose() (which disposes the R3 subscription), then spinner.TriggerInvalidation. Since the subscription was already torn down, OnAnimationInvalidated was never invoked — the _disposed flag's belt-and-suspenders role was not exercised. The test would have passed even if the flag check were absent.

Fix: Keep the original test (renamed AnimationInvalidation_AfterDispose_SubscriptionDisposalUnhooksCallback — it does validate that subscription teardown unhooks the observer, which is real coverage) and add OnAnimationInvalidated_AfterDispose_BailsOutViaDisposedFlag, which uses reflection to invoke OnAnimationInvalidated directly after Dispose(). This simulates an in-flight callback that already passed the Subject's observer-list dispatch and is about to enter the lock when Dispose completes — exactly the scenario the flag protects.

Verification

Confirmed the new reflection-based test correctly catches the regression: temporarily removed the if (_disposed) return; guard and ran the test:

Failed OnAnimationInvalidated_AfterDispose_BailsOutViaDisposedFlag
Expected: null
Actual: System.ObjectDisposedException: Cannot access a disposed object.
   at R3.CompleteState.ThrowObjectDiposedException()
   at R3.Subject\`1.OnNext(T value)
   at Termina.Layout.StreamingTextNode.OnAnimationInvalidated()

Restored the guard — test passes again. Production code is unchanged in this PR.

Test plan

• dotnet build succeeds, 0 warnings
• Full suite (1125 tests) passes
• Thread-safety tests (6) pass
• Negative test: removing _disposed guard makes the new test fail with the expected ObjectDisposedException from R3
• CI green on PR