diff --git a/HellionChat/Plugin.cs b/HellionChat/Plugin.cs
index 60952aa..9204b5c 100755
--- a/HellionChat/Plugin.cs
+++ b/HellionChat/Plugin.cs
@@ -373,6 +373,7 @@ public sealed class Plugin : IAsyncDalamudPlugin
                 new SelfTests.HoverSheenAllocStep(this),
                 new SelfTests.HonorificHeaderRenderStep(this),
                 new SelfTests.PerformanceBaselineStep(this),
+                new SelfTests.MainWindowFocusOpacityStep(this),
             ]);
 
             // Re-surface the wizard for existing users when a major UX
diff --git a/HellionChat/SelfTests/MainWindowFocusOpacityStep.cs b/HellionChat/SelfTests/MainWindowFocusOpacityStep.cs
new file mode 100644
index 0000000..0076e0a
--- /dev/null
+++ b/HellionChat/SelfTests/MainWindowFocusOpacityStep.cs
@@ -0,0 +1,54 @@
+using Dalamud.Bindings.ImGui;
+using Dalamud.Plugin.SelfTest;
+
+namespace HellionChat.SelfTests;
+
+// UI-12 focus opacity. Pins the pure ResolveBgAlpha contract (focused →
+// WindowOpacity, unfocused → WindowOpacityInactive). The PreDraw wiring
+// (BgAlpha = ResolveBgAlpha(IsFocused) behind the main-viewport/!docked guard)
+// is NOT headless-deterministic — the guard may leave BgAlpha null when
+// LastViewport is stale on a /xlperf frame — so the wiring is verified by the
+// reviewer grep (ResolveBgAlpha has a non-test caller: MainWindow.PreDraw) and
+// the visible transparency by in-game smoke, not by driving PreDraw here.
+internal sealed class MainWindowFocusOpacityStep : ISelfTestStep
+{
+    private readonly Plugin plugin;
+
+    public MainWindowFocusOpacityStep(Plugin plugin)
+    {
+        this.plugin = plugin;
+    }
+
+    public string Name => "Hellion Chat - MainWindow focus opacity";
+
+    public SelfTestStepResult RunStep()
+    {
+        var window = this.plugin.MainWindow;
+        if (window is null)
+        {
+            ImGui.Text("Plugin.MainWindow is null");
+            return SelfTestStepResult.Fail;
+        }
+
+        // Contract: focused returns the focused opacity, unfocused the inactive one.
+        if (window.ResolveBgAlpha(true) != Plugin.Config.WindowOpacity)
+        {
+            ImGui.Text(
+                $"ResolveBgAlpha(true) = {window.ResolveBgAlpha(true)}, expected {Plugin.Config.WindowOpacity}"
+            );
+            return SelfTestStepResult.Fail;
+        }
+
+        if (window.ResolveBgAlpha(false) != Plugin.Config.WindowOpacityInactive)
+        {
+            ImGui.Text(
+                $"ResolveBgAlpha(false) = {window.ResolveBgAlpha(false)}, expected {Plugin.Config.WindowOpacityInactive}"
+            );
+            return SelfTestStepResult.Fail;
+        }
+
+        return SelfTestStepResult.Pass;
+    }
+
+    public void CleanUp() { }
+}
diff --git a/HellionChat/Ui/Windows/MainWindow.cs b/HellionChat/Ui/Windows/MainWindow.cs
index d9251ba..cecf4d2 100644
--- a/HellionChat/Ui/Windows/MainWindow.cs
+++ b/HellionChat/Ui/Windows/MainWindow.cs
@@ -37,6 +37,9 @@ internal sealed class MainWindow : Window
     public Vector2 LastWindowSize { get; private set; } = Vector2.Zero;
     internal unsafe ImGuiViewport* LastViewport;
 
+    // 1.5.6 viewport-guard input: tracked in Draw, read by PreDraw next frame.
+    private bool _wasDocked;
+
     public MainWindow(
         Components.HonorificHeader honorific,
         Components.Sidebar sidebar,
@@ -70,6 +73,35 @@ internal sealed class MainWindow : Window
         RespectCloseHotkey = false;
     }
 
+    // UI-12: per-window focus-dependent opacity. ResolveBgAlpha stays guard-free
+    // and pure so the self-test can drive it directly; PreDraw owns the guard +
+    // wiring. 1.5.6 parity (focused → WindowOpacity, unfocused →
+    // WindowOpacityInactive, ChatLogWindow.PreOpenCheck 1d3b429:724).
+    internal float ResolveBgAlpha(bool isFocused) =>
+        isFocused ? Plugin.Config.WindowOpacity : Plugin.Config.WindowOpacityInactive;
+
+    public override void PreDraw()
+    {
+        // Dalamud's WindowHost turns Window.BgAlpha into SetNextWindowBgAlpha
+        // (WindowHost.cs:650-652), which REPLACES this one window's WindowBg
+        // alpha (imgui.cpp:7229). The global GlobalStyleScope clamp is left
+        // untouched, so Settings/DbViewer/popouts/wizard keep today's opacity.
+        // Viewport guard (1.5.6 parity, ChatLogWindow.PreOpenCheck 1d3b429:718):
+        // only drive BgAlpha while the window is on the main viewport and not
+        // docked. On a floated own-viewport (Dalamud multi-viewport mode) the
+        // WindowBg alpha would compose against the OS-layer alpha (double
+        // transparency), so leave BgAlpha null there and let the global scope
+        // govern. LastViewport/_wasDocked are last frame's values from Draw
+        // (one-frame latency, accepted, matches 1.5.6).
+        unsafe
+        {
+            if (LastViewport == ImGuiHelpers.MainViewport.Handle && !_wasDocked)
+                BgAlpha = ResolveBgAlpha(IsFocused);
+            else
+                BgAlpha = null;
+        }
+    }
+
     public Tab? ActiveTab => _activeTab;
 
     // Internal accessors for self-tests so the probes can reach the live
@@ -99,6 +131,7 @@ internal sealed class MainWindow : Window
         {
             LastViewport = ImGui.GetWindowViewport().Handle;
         }
+        _wasDocked = ImGui.IsWindowDocked();
 
         // Primary pool-reset path; InputPreview has a defensive fallback for the MainWindow-closed edge case.
         _handlerLender.ResetCounter();