fix: latestRenderNodes was not thread-safe (fixes #1938)

CHANGELOG.md

@@ -10,6 +10,7 @@ All notable changes to **bUnit** will be documented in this file. The project ad
 
 - `DisposeComponentsAsync()` could dispose nothing when the renderer's dispatcher was busy, e.g. with a timer calling `InvokeAsync`. Reported by [@xfunc-tim-schneider](https://github.com/xfunc-tim-schneider) in #1939. Fixed by [@linkdotnet](https://github.com/linkdotnet).
 - Markup generation and `FindComponents` could recurse until the test host crashed with a stack overflow when a render threw while instantiating a child component. Reported in #1935. Fixed by [@linkdotnet](https://github.com/linkdotnet).
+- `Find()` could keep returning DOM nodes from the previous render when a render, e.g. from a timer, happened while the nodes were parsed, causing `UnknownEventHandlerIdException`. Reported by [@xfunc-tim-schneider](https://github.com/xfunc-tim-schneider) in #1938. Fixed by [@linkdotnet](https://github.com/linkdotnet).
 
 ## [2.11.3] - 2026-09-13
 

src/bunit/Rendering/RenderedComponent.cs

@@ -18,7 +18,7 @@ internal sealed class RenderedComponent<TComponent> : ComponentState, IRenderedC
 	private readonly BunitHtmlParser htmlParser;
 
 	private string markup = string.Empty;
-	private INodeList? latestRenderNodes;
+	private MarkupNodes? latestRenderNodes;
 
 	/// <summary>
 	/// Gets the component under test.
@@ -68,7 +68,17 @@ public INodeList Nodes
 		get
 		{
 			EnsureComponentNotDisposed();
-			return latestRenderNodes ??= htmlParser.Parse(Markup);
+			// A render can finish while Markup is parsed; keying on the markup reference stops that stale parse from being cached (#1938).
+			// Not needed in v3: the shared render tree (#1910) binds nodes to an immutable markup snapshot.
+			var currentMarkup = Markup;
+			var cached = latestRenderNodes;
+			if (cached is null || !ReferenceEquals(cached.Markup, currentMarkup))
+			{
+				cached = new MarkupNodes(currentMarkup, htmlParser.Parse(currentMarkup));
+				latestRenderNodes = cached;
+			}
+
+			return cached.Nodes;
 		}
 	}
 
@@ -169,4 +179,6 @@ public override ValueTask DisposeAsync()
 		Dispose();
 		return base.DisposeAsync();
 	}
+
+	private sealed record MarkupNodes(string Markup, INodeList Nodes);
 }

tests/bunit.tests/Rendering/RenderedComponentTest.cs

@@ -297,6 +297,29 @@ public void Test029()
 		results.ShouldBeEmpty();
 	}
 
+	[Fact(DisplayName = "Nodes reflects the latest markup after a render on another thread (#1938)")]
+	public void Test030()
+	{
+		var cut = Render<Wrapper>(ps => ps.AddChildContent("<p>0</p>"));
+		_ = cut.Nodes;
+		cut.Render(ps => ps.AddChildContent("<p>1</p>"));
+		// Holding the parser's lock pauses the reader after it captured the old markup but before it caches the parsed nodes.
+		var parserLock = typeof(BunitHtmlParser)
+			.GetField("parserLock", BindingFlags.NonPublic | BindingFlags.Instance)!
+			.GetValue(Services.GetRequiredService<BunitHtmlParser>())!;
+		var reader = new Thread(() => _ = cut.Nodes);
+
+		lock (parserLock)
+		{
+			reader.Start();
+			SpinWait.SpinUntil(() => reader.ThreadState.HasFlag(ThreadState.WaitSleepJoin), TimeSpan.FromSeconds(5)).ShouldBeTrue();
+			cut.Render(ps => ps.AddChildContent("<p>2</p>"));
+		}
+
+		reader.Join();
+		cut.Find("p").TextContent.ShouldBe("2");
+	}
+
 	private class BaseComponent : ComponentBase
 	{
 		protected override void BuildRenderTree(RenderTreeBuilder builder)