Fix blank ErrorBoundary when multiple exceptions thrown in one batch

[dariatiurina]

Summary

Fixes ErrorBoundary rendering when multiple child components throw during the same render batch. The renderer now preserves the boundary's error content instead of leaving the boundary blank.

Changes

• Tracks error boundaries with a pending forced subtree-clear render and queues that render only once per boundary.
• Clears the tracking entry when the queued render runs or when the boundary is disposed before the render can run.
• Allows the renderer test boundary helper to receive multiple exception notifications.

The first forced empty render remains ahead of the boundary's error-content render, so the failed subtree is discarded. Suppressing duplicate empty renders prevents a later one from clearing the error content.

Testing

• Adds a unit regression test with three child components throwing during one render batch.
• Adds an end-to-end test with multiple components throwing from a @foreach, verifying that the error UI is displayed without triggering the global error state.

Fixes #56950

[Copilot]

🟡 Changes recommended

Two moderate findings remain, along with a minor test-name nit.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

Fixes blank ErrorBoundary output when multiple child components throw in one render batch.

Changes:

• Coalesces pending subtree-clear renders per error boundary.
• Adds unit and end-to-end regression coverage.
• Updates test helpers for repeated exception notifications.

File summaries

File	Summary	Findings
src/Components/Components/src/RenderTree/Renderer.cs	Tracks pending boundary subtree clears.	Moderate (1 vote): Delayed exceptions after disposal can leave tracking IDs in the renderer.
src/Components/Components/test/ComponentBaseTest.cs	Adds same-batch exception coverage.	Moderate (2 votes): Assertions do not verify the boundary’s error content or render output.
src/Components/test/E2ETest/Tests/ErrorBoundaryTest.cs	Adds end-to-end regression coverage.	Nit (2 votes): Rename ForOnce to CanHandleMultipleExceptionsAtOnce.
src/Components/test/testassets/BasicTestApp/ErrorBoundaryTest/ForeachErrorsChild.razor	Adds a throwing child component fixture.	No findings.
src/Components/test/testassets/BasicTestApp/ErrorBoundaryTest/ErrorBoundaryCases.razor	Adds the foreach error scenario.	No findings.
src/Components/Components/test/RendererTest.cs	Allows repeated exception notifications.	No findings.

Review details

Suppressed comments (1)

src/Components/Components/src/RenderTree/Renderer.cs:1216

• When a delayed exception is routed to a boundary after disposal, AddToRenderQueue returns because the state is no longer registered. Since this Add happens first, the disposal cleanup has already run and cannot remove the newly-added ID; each such boundary leaves an entry in the HashSet for the renderer lifetime. Guard the tracking add with a registered-state check (or make AddToRenderQueue report whether it enqueued).

                if (_errorBoundariesWithPendingSubtreeClear.Add(boundaryComponentId))

• Files reviewed: 6/6 changed files
• Comments generated: 2
• Review effort level: Lite

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