test(selftest): pin the sidebar active-surface invariant

Drives the real Sidebar.Draw and reads the render counter, so a regression in
the draw path fails instead of a parallel calculation passing.

Three cases: one of two tabs active draws exactly one surface, a null active
tab draws zero, and icon-only mode still marks the active row.

Zero is a legitimate state, not a failure -- PickMainActiveTab returns null
when every tab is popped out, so the invariant is "at most one", not "exactly
one".

HellionChat/Plugin.cs

@@ -454,6 +454,7 @@ public sealed class Plugin : IAsyncDalamudPlugin
                 new SelfTests.TellResetOnActivateStep(),
                 new SelfTests.CurrentTabCouplingStep(this),
                 new SelfTests.SidebarUnreadDotStep(this),
+                new SelfTests.SidebarActiveSurfaceStep(this),
                 new SelfTests.UnreadDecisionStep(),
                 new SelfTests.CurrentTabGuidedStep(this),
                 new SelfTests.CardClipPlanStep(this),

HellionChat/SelfTests/SidebarActiveSurfaceStep.cs

@@ -0,0 +1,91 @@
+using Dalamud.Bindings.ImGui;
+using Dalamud.Plugin.SelfTest;
+using HellionChat.Code;
+
+namespace HellionChat.SelfTests;
+
+// v1.10.0/C3: the active row gets a surface and an accent bar, so exactly the
+// row the user is on must be marked -- and only that one. Drives the real
+// Sidebar.Draw and reads the render-observability counter, so a regression in
+// the draw path fails rather than a parallel calculation passing.
+//
+// "At most one", not "exactly one": PickMainActiveTab returns null when every
+// tab is popped out, which is a legitimate state with zero marked rows.
+internal sealed class SidebarActiveSurfaceStep : ISelfTestStep
+{
+    private readonly Plugin _plugin;
+
+    public SidebarActiveSurfaceStep(Plugin plugin) => _plugin = plugin;
+
+    public string Name => "Hellion Chat - Sidebar active surface";
+
+    public SelfTestStepResult RunStep()
+    {
+        var sidebar = _plugin.MainWindow.GetSidebarForSelfTest();
+        if (sidebar is null)
+        {
+            ImGui.Text("Sidebar null");
+            SelfTestReport.Append(Name, "FAIL", new[] { "Sidebar null" });
+            return SelfTestStepResult.Fail;
+        }
+
+        var a = NewProbe("Surface Probe A@SelfTest");
+        var b = NewProbe("Surface Probe B@SelfTest");
+        var list = new List<Tab> { a, b };
+        var width = (float)Plugin.Config.SidebarAutoSwitchThresholdPx + 100f;
+        var savedWidth = Plugin.Config.SidebarWidth;
+
+        try
+        {
+            Plugin.Config.SidebarWidth = 220;
+
+            // (a) one of two tabs is active -> exactly one surface
+            Tab? active = a;
+            sidebar.Draw(width, list, ref active);
+            if (sidebar.LastRenderedActiveSurfaceCount != 1)
+                return Fail(
+                    $"active tab drew {sidebar.LastRenderedActiveSurfaceCount}, expected 1"
+                );
+
+            // (b) no active tab (every tab popped out) -> zero, not a crash
+            active = null;
+            sidebar.Draw(width, list, ref active);
+            if (sidebar.LastRenderedActiveSurfaceCount != 0)
+                return Fail(
+                    $"null active drew {sidebar.LastRenderedActiveSurfaceCount}, expected 0"
+                );
+
+            // (c) icon-only mode still marks the active row
+            active = b;
+            sidebar.Draw((float)Plugin.Config.SidebarAutoSwitchThresholdPx - 1f, list, ref active);
+            if (sidebar.LastRenderedActiveSurfaceCount != 1)
+                return Fail($"icon-only drew {sidebar.LastRenderedActiveSurfaceCount}, expected 1");
+
+            SelfTestReport.Append(Name, "PASS", new[] { "1 / 0 / 1 across the three cases" });
+            return SelfTestStepResult.Pass;
+        }
+        finally
+        {
+            Plugin.Config.SidebarWidth = savedWidth;
+        }
+    }
+
+    private static Tab NewProbe(string name) =>
+        new()
+        {
+            Name = name,
+            SelectedChannels = new Dictionary<ChatType, (ChatSource, ChatSource)>
+            {
+                [ChatType.Say] = (ChatSourceExt.All, ChatSourceExt.All),
+            },
+        };
+
+    private SelfTestStepResult Fail(string message)
+    {
+        ImGui.Text(message);
+        SelfTestReport.Append(Name, "FAIL", new[] { message });
+        return SelfTestStepResult.Fail;
+    }
+
+    public void CleanUp() { }
+}