diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index da47ea2303..e1308f6926 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -2610,6 +2610,7 @@ jobs:
               - 'Darling/Darling.Tests/McpArgumentTypeRows.cs'
               - 'Darling/Darling.Tests/StatementScrubCanary.cs'
               - 'Darling/Darling.Tests/GuardStageScanner.cs'
+              - 'Darling/Darling.Tests/WpfStaGate.cs'
       # How much of the Lite suite THIS leg runs, decided from the filter answers alone (no build needed):
       #   full  - a Lite, Lite.Tests, core or root path changed, or a Darling.Tests file that Lite.Tests
       #           compiles into itself, or the event is not a pull request (a push
diff --git a/Darling/Darling.Tests/AvailabilityGroupsTabRefreshTests.cs b/Darling/Darling.Tests/AvailabilityGroupsTabRefreshTests.cs
index fcda327969..902fc182e2 100644
--- a/Darling/Darling.Tests/AvailabilityGroupsTabRefreshTests.cs
+++ b/Darling/Darling.Tests/AvailabilityGroupsTabRefreshTests.cs
@@ -257,6 +257,7 @@ private static AgTopologyDatabaseRow Database(int serverId, string serverName, s
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/PlanViewerRuntimeSummaryNestingTests.cs b/Darling/Darling.Tests/PlanViewerRuntimeSummaryNestingTests.cs
index 07ab7fcb17..e0d3d665f2 100644
--- a/Darling/Darling.Tests/PlanViewerRuntimeSummaryNestingTests.cs
+++ b/Darling/Darling.Tests/PlanViewerRuntimeSummaryNestingTests.cs
@@ -129,6 +129,7 @@ private static (TextBlock Label, TextBlock Value) RowOf(Grid grid, string label)
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/RawWindowFloorViewerPortTests.cs b/Darling/Darling.Tests/RawWindowFloorViewerPortTests.cs
index 2484e7574a..c12aa5c229 100644
--- a/Darling/Darling.Tests/RawWindowFloorViewerPortTests.cs
+++ b/Darling/Darling.Tests/RawWindowFloorViewerPortTests.cs
@@ -80,6 +80,7 @@ public void UpdateTruncationBanner_StaysCollapsed_WhenTheProbeFoundNoFloorAtAll(
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/StatementScrubViewerPlanWithheldTests.cs b/Darling/Darling.Tests/StatementScrubViewerPlanWithheldTests.cs
index 4c63ed976c..eb9470d889 100644
--- a/Darling/Darling.Tests/StatementScrubViewerPlanWithheldTests.cs
+++ b/Darling/Darling.Tests/StatementScrubViewerPlanWithheldTests.cs
@@ -153,6 +153,7 @@ public void SaveXmlToFile_RefusesAWithheldReportBeforeTheSaveDialog_AndTheViewer
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerBlockedProcessReportsCoverageLiveTests.cs b/Darling/Darling.Tests/ViewerBlockedProcessReportsCoverageLiveTests.cs
index 128119e611..d40a631785 100644
--- a/Darling/Darling.Tests/ViewerBlockedProcessReportsCoverageLiveTests.cs
+++ b/Darling/Darling.Tests/ViewerBlockedProcessReportsCoverageLiveTests.cs
@@ -632,6 +632,7 @@ public async ValueTask DisposeAsync()
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerClutterAndMemoryPressureDataStartTests.cs b/Darling/Darling.Tests/ViewerClutterAndMemoryPressureDataStartTests.cs
index 48d7e34dcb..bf0668e3dc 100644
--- a/Darling/Darling.Tests/ViewerClutterAndMemoryPressureDataStartTests.cs
+++ b/Darling/Darling.Tests/ViewerClutterAndMemoryPressureDataStartTests.cs
@@ -414,6 +414,7 @@ await Assert.ThrowsAsync<InvalidOperationException>(() => ViewerServerTab.AwaitR
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerCollectionHealthLoadTests.cs b/Darling/Darling.Tests/ViewerCollectionHealthLoadTests.cs
index 6ef1b6c785..8f7a04959b 100644
--- a/Darling/Darling.Tests/ViewerCollectionHealthLoadTests.cs
+++ b/Darling/Darling.Tests/ViewerCollectionHealthLoadTests.cs
@@ -77,6 +77,7 @@ private static List<CollectionLogRow> Runs(int count, DateTime first) =>
     private static void OnDispatcher(Func<Task> body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             var dispatcher = Dispatcher.CurrentDispatcher;
diff --git a/Darling/Darling.Tests/ViewerCollectionLogDataStartLiveTests.cs b/Darling/Darling.Tests/ViewerCollectionLogDataStartLiveTests.cs
index 8a87ea3adf..c6bbe3bd29 100644
--- a/Darling/Darling.Tests/ViewerCollectionLogDataStartLiveTests.cs
+++ b/Darling/Darling.Tests/ViewerCollectionLogDataStartLiveTests.cs
@@ -244,6 +244,7 @@ private DrillAnswer Drill(int serverId, string collector, DateTime? asOfUtc)
         var viewer = _store.Viewer!;
         DrillAnswer? answer = null;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             var dispatcher = Dispatcher.CurrentDispatcher;
diff --git a/Darling/Darling.Tests/ViewerCollectorDurationHoverTests.cs b/Darling/Darling.Tests/ViewerCollectorDurationHoverTests.cs
index 5610535441..109c4cd7a8 100644
--- a/Darling/Darling.Tests/ViewerCollectorDurationHoverTests.cs
+++ b/Darling/Darling.Tests/ViewerCollectorDurationHoverTests.cs
@@ -48,6 +48,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try
diff --git a/Darling/Darling.Tests/ViewerConfigChangesDataStartTests.cs b/Darling/Darling.Tests/ViewerConfigChangesDataStartTests.cs
index f1ed7d313c..12dde354dc 100644
--- a/Darling/Darling.Tests/ViewerConfigChangesDataStartTests.cs
+++ b/Darling/Darling.Tests/ViewerConfigChangesDataStartTests.cs
@@ -334,6 +334,7 @@ public static string Since(DateTime utc, TimeDisplayMode mode = TimeDisplayMode.
     public static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerDataStartBannerTests.cs b/Darling/Darling.Tests/ViewerDataStartBannerTests.cs
index afdeb821fe..f0b716aa1b 100644
--- a/Darling/Darling.Tests/ViewerDataStartBannerTests.cs
+++ b/Darling/Darling.Tests/ViewerDataStartBannerTests.cs
@@ -332,6 +332,7 @@ public void TheBanner_NamesItsInstants_OnTheInvariantCulture(string cultureName)
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerDisabledListBoxThemeTests.cs b/Darling/Darling.Tests/ViewerDisabledListBoxThemeTests.cs
index 4a43a1eecd..280d6767c7 100644
--- a/Darling/Darling.Tests/ViewerDisabledListBoxThemeTests.cs
+++ b/Darling/Darling.Tests/ViewerDisabledListBoxThemeTests.cs
@@ -194,6 +194,7 @@ private static ListBox ParseServerList()
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerEventDataStartTests.cs b/Darling/Darling.Tests/ViewerEventDataStartTests.cs
index 04b86763a4..6f20dc297c 100644
--- a/Darling/Darling.Tests/ViewerEventDataStartTests.cs
+++ b/Darling/Darling.Tests/ViewerEventDataStartTests.cs
@@ -454,6 +454,7 @@ public void TheProbes_GoThroughTheSharedFloor_OverTheCollectorTheGridIsNamedFor(
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerJobHistoryDataStartLiveTests.cs b/Darling/Darling.Tests/ViewerJobHistoryDataStartLiveTests.cs
index 295a695e7f..66aa875ff2 100644
--- a/Darling/Darling.Tests/ViewerJobHistoryDataStartLiveTests.cs
+++ b/Darling/Darling.Tests/ViewerJobHistoryDataStartLiveTests.cs
@@ -287,6 +287,7 @@ public async ValueTask DisposeAsync()
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerJobHistoryDataStartTests.cs b/Darling/Darling.Tests/ViewerJobHistoryDataStartTests.cs
index 6bd8b51faa..ba340e3b2a 100644
--- a/Darling/Darling.Tests/ViewerJobHistoryDataStartTests.cs
+++ b/Darling/Darling.Tests/ViewerJobHistoryDataStartTests.cs
@@ -282,6 +282,7 @@ public void AProbeThatThrows_ShowsNoNote_AndTheStepDoesNotThrow()
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerLongQueriesDataStartTests.cs b/Darling/Darling.Tests/ViewerLongQueriesDataStartTests.cs
index 75cda4a675..a6abd5bd58 100644
--- a/Darling/Darling.Tests/ViewerLongQueriesDataStartTests.cs
+++ b/Darling/Darling.Tests/ViewerLongQueriesDataStartTests.cs
@@ -185,6 +185,7 @@ public void AFullPage_RaisesTheNotice_AtItsOldestRow_AndOneRowFewerRaisesNone()
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerPlanCorrectionsDataStartTests.cs b/Darling/Darling.Tests/ViewerPlanCorrectionsDataStartTests.cs
index 1273ee3ac0..e57bf73956 100644
--- a/Darling/Darling.Tests/ViewerPlanCorrectionsDataStartTests.cs
+++ b/Darling/Darling.Tests/ViewerPlanCorrectionsDataStartTests.cs
@@ -183,6 +183,7 @@ public void AFullPage_RaisesTheNotice_AtItsOldestRow_AndOneRowFewerRaisesNone()
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerQueryGridsDataStartLiveTests.cs b/Darling/Darling.Tests/ViewerQueryGridsDataStartLiveTests.cs
index 154b4b4ae8..2d1d888be6 100644
--- a/Darling/Darling.Tests/ViewerQueryGridsDataStartLiveTests.cs
+++ b/Darling/Darling.Tests/ViewerQueryGridsDataStartLiveTests.cs
@@ -547,6 +547,7 @@ public static DateTime NowToTheMinute()
     {
         string? text = null;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             var savedMode = ViewerTimeHelper.CurrentDisplayMode;
diff --git a/Darling/Darling.Tests/ViewerQueryHeatmapDataStartTests.cs b/Darling/Darling.Tests/ViewerQueryHeatmapDataStartTests.cs
index 0375b92fca..969bf554dc 100644
--- a/Darling/Darling.Tests/ViewerQueryHeatmapDataStartTests.cs
+++ b/Darling/Darling.Tests/ViewerQueryHeatmapDataStartTests.cs
@@ -175,6 +175,7 @@ public void AnEmptyChart_NamesTheCoverage_AndAFailedProbeNamesNothing()
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerQueryStoreRegressionsDataStartTests.cs b/Darling/Darling.Tests/ViewerQueryStoreRegressionsDataStartTests.cs
index f43c62be28..60a3250874 100644
--- a/Darling/Darling.Tests/ViewerQueryStoreRegressionsDataStartTests.cs
+++ b/Darling/Darling.Tests/ViewerQueryStoreRegressionsDataStartTests.cs
@@ -197,6 +197,7 @@ public void AProbeThatThrows_HidesTheBanner_InsteadOfThrowing()
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerScreenReaderNamesFollowUpTests.cs b/Darling/Darling.Tests/ViewerScreenReaderNamesFollowUpTests.cs
index 3fb0fe6638..8e6a4488a4 100644
--- a/Darling/Darling.Tests/ViewerScreenReaderNamesFollowUpTests.cs
+++ b/Darling/Darling.Tests/ViewerScreenReaderNamesFollowUpTests.cs
@@ -33,6 +33,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Darling/Darling.Tests/ViewerScreenReaderNamesTests.cs b/Darling/Darling.Tests/ViewerScreenReaderNamesTests.cs
index 343c1b8ec2..f4fdf9cc41 100644
--- a/Darling/Darling.Tests/ViewerScreenReaderNamesTests.cs
+++ b/Darling/Darling.Tests/ViewerScreenReaderNamesTests.cs
@@ -36,6 +36,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Darling/Darling.Tests/ViewerScreenReaderRowNamesTests.cs b/Darling/Darling.Tests/ViewerScreenReaderRowNamesTests.cs
index df970af7d5..94f77c5620 100644
--- a/Darling/Darling.Tests/ViewerScreenReaderRowNamesTests.cs
+++ b/Darling/Darling.Tests/ViewerScreenReaderRowNamesTests.cs
@@ -31,6 +31,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Darling/Darling.Tests/ViewerShortWindowDataStartTests.cs b/Darling/Darling.Tests/ViewerShortWindowDataStartTests.cs
index 51c430a0ae..17d89db876 100644
--- a/Darling/Darling.Tests/ViewerShortWindowDataStartTests.cs
+++ b/Darling/Darling.Tests/ViewerShortWindowDataStartTests.cs
@@ -249,6 +249,7 @@ public void EveryQueriesTabFloorRead_GoesThroughTheViewersMethod_NeverTheSharedP
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/ViewerStatsRowBlankCellTests.cs b/Darling/Darling.Tests/ViewerStatsRowBlankCellTests.cs
index d5c4a4cb23..232bb21e85 100644
--- a/Darling/Darling.Tests/ViewerStatsRowBlankCellTests.cs
+++ b/Darling/Darling.Tests/ViewerStatsRowBlankCellTests.cs
@@ -151,6 +151,7 @@ private static string CellText(object row, string path)
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Darling/Darling.Tests/WpfStaGate.cs b/Darling/Darling.Tests/WpfStaGate.cs
new file mode 100644
index 0000000000..e03e81b526
--- /dev/null
+++ b/Darling/Darling.Tests/WpfStaGate.cs
@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2026 Erik Darling, Darling Data LLC
+ *
+ * This file is part of the SQL Server Performance Monitor.
+ *
+ * Licensed under the MIT License. See LICENSE file in the project root for full license information.
+ */
+
+using System;
+using System.Threading;
+
+/// <summary>
+/// #5559: one gate for every test that builds WPF elements on its own STA thread. WPF keeps process-wide state that is
+/// not thread-safe: <c>DependencyPropertyDescriptor.FromProperty</c> hands every caller the same descriptor out of a static
+/// cache, and that descriptor's <c>AddValueChanged</c> writes an unsynchronised per-descriptor <c>Dictionary</c>. The app has
+/// one UI thread, so the product never races. xUnit runs test classes in parallel and about fifty of them start their own
+/// STA thread, so two of them reaching <c>AccessibleNames.ActivateCell</c> at once corrupted that dictionary
+/// ("Operations that change non-concurrent collections must have exclusive access") in CI, a different class each time.
+/// <para>
+/// Every STA helper calls <c>using var staGate = WpfStaGate.Enter();</c> on the CALLING thread, before it creates the
+/// thread, and keeps it until it has joined the thread. Only one STA test body then runs at a time. Taking the gate on the
+/// calling thread, not inside the new thread, keeps a timed <c>Join</c> honest: the wait for the gate is not charged to it.
+/// <c>WpfStaGateCensusTests</c> fails when a test file starts an STA thread without calling <see cref="Enter"/>.
+/// </para>
+/// This file lives in Darling.Tests and is compiled into Lite.Tests by a link (one gate, two suites). It is in the global
+/// namespace so the Lite classes, which sit in a different namespace, need no extra <c>using</c>.
+/// </summary>
+internal static class WpfStaGate
+{
+    /* A SemaphoreSlim, not a Monitor: the holder may be released from a different thread than took it if a helper is async. */
+    private static readonly SemaphoreSlim s_gate = new(1, 1);
+
+    /// <summary>Waits for the gate and returns the token that releases it. Not re-entrant: do not call it from inside a body that already holds it.</summary>
+    internal static IDisposable Enter()
+    {
+        s_gate.Wait();
+        return new Releaser();
+    }
+
+    private sealed class Releaser : IDisposable
+    {
+        private int _released;
+
+        public void Dispose()
+        {
+            if (Interlocked.Exchange(ref _released, 1) == 0)
+            {
+                s_gate.Release();
+            }
+        }
+    }
+}
diff --git a/Darling/Darling.Tests/WpfStaGateCensusTests.cs b/Darling/Darling.Tests/WpfStaGateCensusTests.cs
new file mode 100644
index 0000000000..9dfd972fd8
--- /dev/null
+++ b/Darling/Darling.Tests/WpfStaGateCensusTests.cs
@@ -0,0 +1,114 @@
+/*
+ * Copyright (c) 2026 Erik Darling, Darling Data LLC
+ *
+ * This file is part of the SQL Server Performance Monitor.
+ *
+ * Licensed under the MIT License. See LICENSE file in the project root for full license information.
+ */
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Runtime.CompilerServices;
+using System.Text.RegularExpressions;
+using System.Threading;
+using Xunit;
+
+namespace Darling.Tests;
+
+/// <summary>
+/// #5559: every Darling.Tests test that starts its own STA thread takes <see cref="WpfStaGate"/> first. WPF's
+/// <c>DependencyPropertyDescriptor</c> cache hands every STA thread in the process the same descriptor, and that descriptor's
+/// <c>AddValueChanged</c> writes an unsynchronised dictionary, so two test classes in parallel corrupted it in CI. A new
+/// test file that starts an STA thread without the gate brings the flake back, so this fails and names the file.
+/// </summary>
+[Trait("Stage", "Guard")]
+public sealed class WpfStaGateCensusTests
+{
+    /* Built from parts so this file does not contain the text it scans for. */
+    private static readonly string s_sta = "ApartmentState" + ".STA";
+    private const string Gate = "WpfStaGate.Enter()";
+
+    /// <summary>Anti-vacuity: the suite has well over this many STA files, so a scan that stopped seeing them fails.</summary>
+    private const int StaFileFloor = 20;
+
+    [Fact]
+    public void EveryStaThreadStart_IsPrecededByItsOwnGateEntry()
+    {
+        var missing = new List<string>();
+        var staFiles = 0;
+        foreach (var file in Directory.EnumerateFiles(ProjectDir(), "*.cs", SearchOption.AllDirectories))
+        {
+            var rel = Path.GetRelativePath(ProjectDir(), file);
+            if (rel.StartsWith("bin" + Path.DirectorySeparatorChar, StringComparison.Ordinal)
+                || rel.StartsWith("obj" + Path.DirectorySeparatorChar, StringComparison.Ordinal)
+                || Path.GetFileName(file) is "WpfStaGate.cs" or "WpfStaGateCensusTests.cs")
+            {
+                continue;
+            }
+
+            var text = File.ReadAllText(file);
+            if (!text.Contains(s_sta, StringComparison.Ordinal))
+            {
+                continue;
+            }
+
+            staFiles++;
+
+            /* Each STA start needs its OWN gate entry after the previous start: Enter, thread, STA, Enter, thread, STA. */
+            var starts = Regex.Matches(text, Regex.Escape(s_sta)).Select(m => m.Index).ToList();
+            var gates = Regex.Matches(text, Regex.Escape(Gate)).Select(m => m.Index).ToList();
+            var previous = -1;
+            foreach (var start in starts)
+            {
+                if (!gates.Any(g => g > previous && g < start))
+                {
+                    missing.Add(rel);
+                    break;
+                }
+
+                previous = start;
+            }
+        }
+
+        Assert.True(
+            missing.Count == 0,
+            "These test files start an STA thread without taking WpfStaGate first (put `using var staGate = WpfStaGate.Enter();` "
+            + "before the helper creates the thread, #5559): " + string.Join(", ", missing));
+        Assert.True(staFiles >= StaFileFloor, $"only {staFiles} STA test files were found; the scan has stopped seeing them");
+    }
+
+    [Fact]
+    public void TheGate_LetsOnlyOneHolderIn_AtATime()
+    {
+        var inside = 0;
+        var most = 0;
+        var threads = Enumerable.Range(0, 3).Select(_ => new Thread(() =>
+        {
+            for (var i = 0; i < 10; i++)
+            {
+                using var staGate = WpfStaGate.Enter();
+                var now = Interlocked.Increment(ref inside);
+                InterlockedMax(ref most, now);
+                Thread.Sleep(1);
+                Interlocked.Decrement(ref inside);
+            }
+        })).ToList();
+
+        threads.ForEach(t => t.Start());
+        threads.ForEach(t => Assert.True(t.Join(TimeSpan.FromSeconds(30)), "a gate holder never finished"));
+
+        Assert.Equal(1, most);
+    }
+
+    private static void InterlockedMax(ref int target, int value)
+    {
+        int seen;
+        while ((seen = Volatile.Read(ref target)) < value && Interlocked.CompareExchange(ref target, value, seen) != seen)
+        {
+        }
+    }
+
+    private static string ProjectDir([CallerFilePath] string thisFile = "") => Path.GetDirectoryName(thisFile)!;
+}
diff --git a/Lite.Tests/AlertHistoryCountFilterTests.cs b/Lite.Tests/AlertHistoryCountFilterTests.cs
index d1563b8553..9895ee033c 100644
--- a/Lite.Tests/AlertHistoryCountFilterTests.cs
+++ b/Lite.Tests/AlertHistoryCountFilterTests.cs
@@ -52,6 +52,7 @@ public void AColumnFilter_RebuildsTheCountTextFromWhatTheGridNowShows_AndKeepsTh
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Lite.Tests/AvailabilityGroupsTabRefreshTests.cs b/Lite.Tests/AvailabilityGroupsTabRefreshTests.cs
index ebf0977925..a1947c7260 100644
--- a/Lite.Tests/AvailabilityGroupsTabRefreshTests.cs
+++ b/Lite.Tests/AvailabilityGroupsTabRefreshTests.cs
@@ -193,6 +193,7 @@ private static List<AgTopologyCard> BuildTopology(int agCount, int replicasPerAg
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Lite.Tests/BlockingChartsDataStartTests.cs b/Lite.Tests/BlockingChartsDataStartTests.cs
index 9faf3ab8ef..6636692a2c 100644
--- a/Lite.Tests/BlockingChartsDataStartTests.cs
+++ b/Lite.Tests/BlockingChartsDataStartTests.cs
@@ -527,6 +527,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/CollectionLogDrillDataStartTests.cs b/Lite.Tests/CollectionLogDrillDataStartTests.cs
index 21ae723631..756ec4a971 100644
--- a/Lite.Tests/CollectionLogDrillDataStartTests.cs
+++ b/Lite.Tests/CollectionLogDrillDataStartTests.cs
@@ -405,6 +405,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/CollectorDurationHoverTests.cs b/Lite.Tests/CollectorDurationHoverTests.cs
index d95c873e7d..910ce1a93e 100644
--- a/Lite.Tests/CollectorDurationHoverTests.cs
+++ b/Lite.Tests/CollectorDurationHoverTests.cs
@@ -226,6 +226,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/CpuHoverStoredWallClockTests.cs b/Lite.Tests/CpuHoverStoredWallClockTests.cs
index c944719b9c..d1ae07cce3 100644
--- a/Lite.Tests/CpuHoverStoredWallClockTests.cs
+++ b/Lite.Tests/CpuHoverStoredWallClockTests.cs
@@ -427,6 +427,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/CustomTabHeaderInkTests.cs b/Lite.Tests/CustomTabHeaderInkTests.cs
index f1ef736e6d..da8e733793 100644
--- a/Lite.Tests/CustomTabHeaderInkTests.cs
+++ b/Lite.Tests/CustomTabHeaderInkTests.cs
@@ -181,6 +181,7 @@ public void ViewerCollectorHealth_UsesLiveReferences_NoOneTimeCopy()
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Lite.Tests/DataStartBannerBlockingEventsTests.cs b/Lite.Tests/DataStartBannerBlockingEventsTests.cs
index 0384d350dc..80405fb1e6 100644
--- a/Lite.Tests/DataStartBannerBlockingEventsTests.cs
+++ b/Lite.Tests/DataStartBannerBlockingEventsTests.cs
@@ -312,6 +312,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/DataStartBannerLatchSpinlockTests.cs b/Lite.Tests/DataStartBannerLatchSpinlockTests.cs
index 7ada5cf774..3a9e51ef52 100644
--- a/Lite.Tests/DataStartBannerLatchSpinlockTests.cs
+++ b/Lite.Tests/DataStartBannerLatchSpinlockTests.cs
@@ -175,6 +175,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/DataStartBannerProbeFailureTests.cs b/Lite.Tests/DataStartBannerProbeFailureTests.cs
index 4f87f98d93..d2c5189124 100644
--- a/Lite.Tests/DataStartBannerProbeFailureTests.cs
+++ b/Lite.Tests/DataStartBannerProbeFailureTests.cs
@@ -170,6 +170,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/DataStartBannerQueriesTabTests.cs b/Lite.Tests/DataStartBannerQueriesTabTests.cs
index c3091390ee..7dbc1bbebc 100644
--- a/Lite.Tests/DataStartBannerQueriesTabTests.cs
+++ b/Lite.Tests/DataStartBannerQueriesTabTests.cs
@@ -1294,6 +1294,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/DataStartBannerSurfaceTests.cs b/Lite.Tests/DataStartBannerSurfaceTests.cs
index d55ad23bca..16c7bc050e 100644
--- a/Lite.Tests/DataStartBannerSurfaceTests.cs
+++ b/Lite.Tests/DataStartBannerSurfaceTests.cs
@@ -1389,6 +1389,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/DataStartBannerTests.cs b/Lite.Tests/DataStartBannerTests.cs
index 08d55983b8..ed5d2bdb82 100644
--- a/Lite.Tests/DataStartBannerTests.cs
+++ b/Lite.Tests/DataStartBannerTests.cs
@@ -393,6 +393,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/DisplayZoneRendererTests.cs b/Lite.Tests/DisplayZoneRendererTests.cs
index db3260eca7..7cba1f1f09 100644
--- a/Lite.Tests/DisplayZoneRendererTests.cs
+++ b/Lite.Tests/DisplayZoneRendererTests.cs
@@ -33,6 +33,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/JobHistoryDataStartTests.cs b/Lite.Tests/JobHistoryDataStartTests.cs
index 9f928f5e51..727795b9b4 100644
--- a/Lite.Tests/JobHistoryDataStartTests.cs
+++ b/Lite.Tests/JobHistoryDataStartTests.cs
@@ -639,6 +639,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/Lite.Tests.csproj b/Lite.Tests/Lite.Tests.csproj
index 6753fc6ed0..6134f2a376 100644
--- a/Lite.Tests/Lite.Tests.csproj
+++ b/Lite.Tests/Lite.Tests.csproj
@@ -50,6 +50,8 @@
     <Compile Include="..\Darling\Darling.Tests\StatementScrubCanary.cs" Link="StatementScrubCanary.cs" />
     <!-- #5459: the guard-stage scanner, so both suites tag their guard classes by one detector. -->
     <Compile Include="..\Darling\Darling.Tests\GuardStageScanner.cs" Link="GuardStageScanner.cs" />
+    <!-- #5559: the one gate every STA test helper takes, so no two test classes touch WPF's shared descriptor caches at once. -->
+    <Compile Include="..\Darling\Darling.Tests\WpfStaGate.cs" Link="WpfStaGate.cs" />
   </ItemGroup>
 
   <ItemGroup>
diff --git a/Lite.Tests/MainWindowAccessKeyTests.cs b/Lite.Tests/MainWindowAccessKeyTests.cs
index 9b5cb5bffa..7e5fd1ff2c 100644
--- a/Lite.Tests/MainWindowAccessKeyTests.cs
+++ b/Lite.Tests/MainWindowAccessKeyTests.cs
@@ -287,6 +287,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/OverviewBlockingLaneDataStartLiveTests.cs b/Lite.Tests/OverviewBlockingLaneDataStartLiveTests.cs
index e949d279ed..1271e81ff0 100644
--- a/Lite.Tests/OverviewBlockingLaneDataStartLiveTests.cs
+++ b/Lite.Tests/OverviewBlockingLaneDataStartLiveTests.cs
@@ -355,6 +355,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/PlanViewer4632FallbackTests.cs b/Lite.Tests/PlanViewer4632FallbackTests.cs
index 28bb9d97e2..1c438988e4 100644
--- a/Lite.Tests/PlanViewer4632FallbackTests.cs
+++ b/Lite.Tests/PlanViewer4632FallbackTests.cs
@@ -164,6 +164,7 @@ private static SolidColorBrush CriticalOrangeBrushOf(PlanViewerControl control)
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Lite.Tests/QueryWindowTruncationTests.cs b/Lite.Tests/QueryWindowTruncationTests.cs
index 947a2fca01..5d8221472a 100644
--- a/Lite.Tests/QueryWindowTruncationTests.cs
+++ b/Lite.Tests/QueryWindowTruncationTests.cs
@@ -610,6 +610,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/ScreenReaderNamesFollowUpTests.cs b/Lite.Tests/ScreenReaderNamesFollowUpTests.cs
index d03e34d3bb..30d3ab802e 100644
--- a/Lite.Tests/ScreenReaderNamesFollowUpTests.cs
+++ b/Lite.Tests/ScreenReaderNamesFollowUpTests.cs
@@ -34,6 +34,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/ScreenReaderNamesTests.cs b/Lite.Tests/ScreenReaderNamesTests.cs
index 5a32453b9d..6a62235f7b 100644
--- a/Lite.Tests/ScreenReaderNamesTests.cs
+++ b/Lite.Tests/ScreenReaderNamesTests.cs
@@ -36,6 +36,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/ScreenReaderRowNamesTests.cs b/Lite.Tests/ScreenReaderRowNamesTests.cs
index be89a32ce5..616eeb162f 100644
--- a/Lite.Tests/ScreenReaderRowNamesTests.cs
+++ b/Lite.Tests/ScreenReaderRowNamesTests.cs
@@ -32,6 +32,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/ServerTabEmptyStateAndNamesTests.cs b/Lite.Tests/ServerTabEmptyStateAndNamesTests.cs
index 68839582f3..e9c727a0ef 100644
--- a/Lite.Tests/ServerTabEmptyStateAndNamesTests.cs
+++ b/Lite.Tests/ServerTabEmptyStateAndNamesTests.cs
@@ -31,6 +31,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/StandalonePlanViewerPlusTabTests.cs b/Lite.Tests/StandalonePlanViewerPlusTabTests.cs
index a38b1ff3df..b4830ad9be 100644
--- a/Lite.Tests/StandalonePlanViewerPlusTabTests.cs
+++ b/Lite.Tests/StandalonePlanViewerPlusTabTests.cs
@@ -287,6 +287,7 @@ public void UiaSelect(TabItem tab)
     private static void OnStaThread(Action<Rig> body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try
diff --git a/Lite.Tests/StatementScrubLivePlanDisplayTests.cs b/Lite.Tests/StatementScrubLivePlanDisplayTests.cs
index e6698a7d4d..97c7f036e5 100644
--- a/Lite.Tests/StatementScrubLivePlanDisplayTests.cs
+++ b/Lite.Tests/StatementScrubLivePlanDisplayTests.cs
@@ -265,6 +265,7 @@ public void SaveXmlToFile_RefusesAWithheldReportBeforeTheSaveDialog_AndLitesDown
     private static void OnStaThread(Action body)
     {
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { body(); }
diff --git a/Lite.Tests/ThemeColorOverrideTests.cs b/Lite.Tests/ThemeColorOverrideTests.cs
index aae6dd81ef..7741348822 100644
--- a/Lite.Tests/ThemeColorOverrideTests.cs
+++ b/Lite.Tests/ThemeColorOverrideTests.cs
@@ -999,6 +999,7 @@ private static T OnStaThread<T>(Func<T> body)
     {
         T result = default!;
         Exception? error = null;
+        using var staGate = WpfStaGate.Enter();
         var thread = new Thread(() =>
         {
             try { result = body(); }
diff --git a/Lite.Tests/WpfStaGateCensusTests.cs b/Lite.Tests/WpfStaGateCensusTests.cs
new file mode 100644
index 0000000000..e1c268f9db
--- /dev/null
+++ b/Lite.Tests/WpfStaGateCensusTests.cs
@@ -0,0 +1,114 @@
+/*
+ * Copyright (c) 2026 Erik Darling, Darling Data LLC
+ *
+ * This file is part of the SQL Server Performance Monitor.
+ *
+ * Licensed under the MIT License. See LICENSE file in the project root for full license information.
+ */
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Runtime.CompilerServices;
+using System.Text.RegularExpressions;
+using System.Threading;
+using Xunit;
+
+namespace PerformanceMonitorLite.Tests;
+
+/// <summary>
+/// #5559: every Lite.Tests test that starts its own STA thread takes <see cref="WpfStaGate"/> first. WPF's
+/// <c>DependencyPropertyDescriptor</c> cache hands every STA thread in the process the same descriptor, and that descriptor's
+/// <c>AddValueChanged</c> writes an unsynchronised dictionary, so two test classes in parallel corrupted it in CI. A new
+/// test file that starts an STA thread without the gate brings the flake back, so this fails and names the file.
+/// </summary>
+[Trait("Stage", "Guard")]
+public sealed class WpfStaGateCensusTests
+{
+    /* Built from parts so this file does not contain the text it scans for. */
+    private static readonly string s_sta = "ApartmentState" + ".STA";
+    private const string Gate = "WpfStaGate.Enter()";
+
+    /// <summary>Anti-vacuity: the suite has well over this many STA files, so a scan that stopped seeing them fails.</summary>
+    private const int StaFileFloor = 20;
+
+    [Fact]
+    public void EveryStaThreadStart_IsPrecededByItsOwnGateEntry()
+    {
+        var missing = new List<string>();
+        var staFiles = 0;
+        foreach (var file in Directory.EnumerateFiles(ProjectDir(), "*.cs", SearchOption.AllDirectories))
+        {
+            var rel = Path.GetRelativePath(ProjectDir(), file);
+            if (rel.StartsWith("bin" + Path.DirectorySeparatorChar, StringComparison.Ordinal)
+                || rel.StartsWith("obj" + Path.DirectorySeparatorChar, StringComparison.Ordinal)
+                || Path.GetFileName(file) is "WpfStaGate.cs" or "WpfStaGateCensusTests.cs")
+            {
+                continue;
+            }
+
+            var text = File.ReadAllText(file);
+            if (!text.Contains(s_sta, StringComparison.Ordinal))
+            {
+                continue;
+            }
+
+            staFiles++;
+
+            /* Each STA start needs its OWN gate entry after the previous start: Enter, thread, STA, Enter, thread, STA. */
+            var starts = Regex.Matches(text, Regex.Escape(s_sta)).Select(m => m.Index).ToList();
+            var gates = Regex.Matches(text, Regex.Escape(Gate)).Select(m => m.Index).ToList();
+            var previous = -1;
+            foreach (var start in starts)
+            {
+                if (!gates.Any(g => g > previous && g < start))
+                {
+                    missing.Add(rel);
+                    break;
+                }
+
+                previous = start;
+            }
+        }
+
+        Assert.True(
+            missing.Count == 0,
+            "These test files start an STA thread without taking WpfStaGate first (put `using var staGate = WpfStaGate.Enter();` "
+            + "before the helper creates the thread, #5559): " + string.Join(", ", missing));
+        Assert.True(staFiles >= StaFileFloor, $"only {staFiles} STA test files were found; the scan has stopped seeing them");
+    }
+
+    [Fact]
+    public void TheGate_LetsOnlyOneHolderIn_AtATime()
+    {
+        var inside = 0;
+        var most = 0;
+        var threads = Enumerable.Range(0, 3).Select(_ => new Thread(() =>
+        {
+            for (var i = 0; i < 10; i++)
+            {
+                using var staGate = WpfStaGate.Enter();
+                var now = Interlocked.Increment(ref inside);
+                InterlockedMax(ref most, now);
+                Thread.Sleep(1);
+                Interlocked.Decrement(ref inside);
+            }
+        })).ToList();
+
+        threads.ForEach(t => t.Start());
+        threads.ForEach(t => Assert.True(t.Join(TimeSpan.FromSeconds(30)), "a gate holder never finished"));
+
+        Assert.Equal(1, most);
+    }
+
+    private static void InterlockedMax(ref int target, int value)
+    {
+        int seen;
+        while ((seen = Volatile.Read(ref target)) < value && Interlocked.CompareExchange(ref target, value, seen) != seen)
+        {
+        }
+    }
+
+    private static string ProjectDir([CallerFilePath] string thisFile = "") => Path.GetDirectoryName(thisFile)!;
+}