Harden RendererSynchronizationContext restoration

[javiercn]

Why

Fixes #69323.

When queued renderer work is submitted without a flowed ExecutionContext, its continuation can inline onto the thread that releases the renderer queue. PostAsync then installs the renderer synchronization context on that caller-owned thread without an execution-context or thread-pool boundary to restore it. Subsequent CheckAccess calls can incorrectly succeed and bypass dispatcher serialization.

The triggering topology is outside normal Blazor guidance, but restoring the caller's synchronization context is inexpensive defensive hardening.

What changed

• Complete the renderer queue marker while RendererSynchronizationContext is still current, preventing queued continuations from inlining onto the caller's thread.
• Restore the caller's original synchronization context in an inner finally block.
• Add a deterministic unit regression for queued work submitted with execution-context flow suppressed.
• Add server component E2E coverage proving the component callback executes and does not overlap an occupied dispatcher.
• No public API changes.

How to verify

• . .\activate.ps1; dotnet test src\Components\Components\test\Microsoft.AspNetCore.Components.Tests.csproj --no-restore --filter "FullyQualifiedName~RendererSynchronizationContextTest" -v:q -p:UseIisNativeAssets=false
  • 29 passed
• . .\activate.ps1; dotnet test src\Components\test\E2ETest\Microsoft.AspNetCore.Components.E2ETests.csproj --no-restore --filter "FullyQualifiedName~ServerComponentRenderingTest.RestoresContextWhenQueuedWorkDoesNotFlowExecutionContext" -v:q -p:UseIisNativeAssets=false -p:EnforceE2ETestPrerequisites=true
  • 1 passed
• The new unit regression failed against the unmodified implementation.
• An interactive InteractiveServer component repro reported Context leaked: False and Dispatcher overlapped: False twice with the change.

Risks / follow-ups

Completing the queue marker while the renderer context is current can add an asynchronous scheduling hop where a continuation previously inlined. This aligns the affected async/Send path with the synchronous dispatcher overloads, which already complete their builders before restoring the context.

No follow-up is currently required.

[Copilot]

🟢 Approval recommended

No unresolved issues were identified, and unit and E2E coverage validate the fix.

Pull request overview

Hardens RendererSynchronizationContext restoration to prevent context leakage and dispatcher bypass when execution-context flow is suppressed.

Changes:

• Reorders queue-marker completion and context restoration.
• Adds deterministic unit regression coverage.
• Adds server component E2E coverage.

File summaries

File	Description
src/Components/test/testassets/BasicTestApp/DispatchingComponent.razor	Adds the suppressed-flow regression scenario.
src/Components/test/E2ETest/ServerExecutionTests/ServerComponentRenderingTest.cs	Verifies context restoration and dispatcher serialization.
src/Components/Components/test/Rendering/RendererSynchronizationContextTest.cs	Adds focused unit regression coverage.
src/Components/Components/src/Rendering/RendererSynchronizationContext.cs	Safely orders queue completion and context restoration.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Lite

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