test(selftest): pin the top-tab underline invariant

TopTabBar had no observability at all -- no counter, no self-test reaching it.
It now exposes LastRenderedUnderlineCount and MainWindow hands the component
out the same way it already does for the sidebar.

Three cases: one of two tabs active draws exactly one underline, a null active
tab draws zero, and an active tab that is not in the list also draws zero. The
last one matters because the strip skips popped-out tabs, so the active tab
legitimately need not be among the drawn ones.

HellionChat/Plugin.cs

@@ -455,6 +455,7 @@ public sealed class Plugin : IAsyncDalamudPlugin
                 new SelfTests.CurrentTabCouplingStep(this),
                 new SelfTests.SidebarUnreadDotStep(this),
                 new SelfTests.SidebarActiveSurfaceStep(this),
+                new SelfTests.TopTabUnderlineStep(this),
                 new SelfTests.UnreadDecisionStep(),
                 new SelfTests.CurrentTabGuidedStep(this),
                 new SelfTests.CardClipPlanStep(this),

HellionChat/SelfTests/TopTabUnderlineStep.cs

@@ -0,0 +1,76 @@
+using Dalamud.Bindings.ImGui;
+using Dalamud.Plugin.SelfTest;
+using HellionChat.Code;
+
+namespace HellionChat.SelfTests;
+
+// v1.10.0/D1: the top-tab strip marks the active tab with a fill plus an accent
+// underline. Drives the real TopTabBar.Draw and reads the render counter.
+//
+// "At most one", not "exactly one": the strip skips popped-out tabs, so zero
+// underlines is a legitimate state.
+internal sealed class TopTabUnderlineStep : ISelfTestStep
+{
+    private readonly Plugin _plugin;
+
+    public TopTabUnderlineStep(Plugin plugin) => _plugin = plugin;
+
+    public string Name => "Hellion Chat - Top tab underline";
+
+    public SelfTestStepResult RunStep()
+    {
+        var strip = _plugin.MainWindow.GetTopTabsForSelfTest();
+        if (strip is null)
+        {
+            ImGui.Text("TopTabBar null");
+            SelfTestReport.Append(Name, "FAIL", new[] { "TopTabBar null" });
+            return SelfTestStepResult.Fail;
+        }
+
+        var a = NewProbe("Underline Probe A@SelfTest");
+        var b = NewProbe("Underline Probe B@SelfTest");
+        var list = new List<Tab> { a, b };
+
+        // (a) one of two tabs active -> exactly one underline
+        Tab? active = a;
+        strip.Draw(list, ref active);
+        if (strip.LastRenderedUnderlineCount != 1)
+            return Fail(
+                $"active tab drew {strip.LastRenderedUnderlineCount} underlines, expected 1"
+            );
+
+        // (b) no active tab -> zero, not a crash
+        active = null;
+        strip.Draw(list, ref active);
+        if (strip.LastRenderedUnderlineCount != 0)
+            return Fail($"null active drew {strip.LastRenderedUnderlineCount}, expected 0");
+
+        // (c) an active tab that is not in the list -> still zero
+        active = NewProbe("Absent@SelfTest");
+        strip.Draw(list, ref active);
+        if (strip.LastRenderedUnderlineCount != 0)
+            return Fail($"absent active drew {strip.LastRenderedUnderlineCount}, expected 0");
+
+        SelfTestReport.Append(Name, "PASS", new[] { "1 / 0 / 0 across the three cases" });
+        return SelfTestStepResult.Pass;
+    }
+
+    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() { }
+}

HellionChat/Ui/Windows/MainWindow.cs

@@ -204,6 +204,8 @@ internal sealed class MainWindow : Window, IFocusableChatWindow
 
     internal Components.MessageList GetMessageListForSelfTest() => _messages;
 
+    internal Components.TopTabBar GetTopTabsForSelfTest() => _topTabs;
+
     public override bool DrawConditions() => !_userHidden;
 
     internal void UserHide() => _userHidden = true;