diff --git a/HellionChat/Configuration.cs b/HellionChat/Configuration.cs
index 1b1524b..ae7a9b8 100755
--- a/HellionChat/Configuration.cs
+++ b/HellionChat/Configuration.cs
@@ -44,6 +44,10 @@ public class Configuration : IPluginConfiguration
     // Global window opacity, applied across all themes.
     public float WindowOpacity = 0.85f;
 
+    // UI-12: background opacity of the main chat window while unfocused.
+    // WindowOpacity above stays the focused value.
+    public float WindowOpacityInactive = 0.65f;
+
     // Reserved for future UI toggles; pre-declared to avoid a migration later.
     public bool ReduceMotion;
 
@@ -360,6 +364,7 @@ public class Configuration : IPluginConfiguration
         // v1.1.0 theme engine fields
         Theme = other.Theme;
         WindowOpacity = other.WindowOpacity;
+        WindowOpacityInactive = other.WindowOpacityInactive;
         ReduceMotion = other.ReduceMotion;
         UseCompactDensity = other.UseCompactDensity;
 
diff --git a/HellionChat/Ui/ChatLogWindow.cs b/HellionChat/Ui/ChatLogWindow.cs
index 072cb5f..227124b 100644
--- a/HellionChat/Ui/ChatLogWindow.cs
+++ b/HellionChat/Ui/ChatLogWindow.cs
@@ -709,7 +709,15 @@ public sealed class ChatLogWindow : Window
         // Window-Deckkraft eingestellt hat, hat dieses Per-Window-Override
         // Vorrang über unseren Slider — wir dokumentieren das im HelpMarker.
         if (LastViewport == ImGuiHelpers.MainViewport.Handle && !WasDocked)
-            BgAlpha = Plugin.Config.WindowOpacity;
+        {
+            // UI-12: focus-dependent opacity. PreOpenCheck runs before Begin();
+            // Window.IsFocused holds last frame's RootAndChildWindows focus, set
+            // by Dalamud's WindowHost after Begin(). One-frame latency is
+            // accepted.
+            BgAlpha = IsFocused
+                ? Plugin.Config.WindowOpacity
+                : Plugin.Config.WindowOpacityInactive;
+        }
 
         LastViewport = ImGui.GetWindowViewport().Handle;
         WasDocked = ImGui.IsWindowDocked();
diff --git a/HellionChat/Ui/SettingsTabs/ThemeAndLayout.cs b/HellionChat/Ui/SettingsTabs/ThemeAndLayout.cs
index ccc1e62..f5a9f9e 100644
--- a/HellionChat/Ui/SettingsTabs/ThemeAndLayout.cs
+++ b/HellionChat/Ui/SettingsTabs/ThemeAndLayout.cs
@@ -296,6 +296,24 @@ internal sealed class ThemeAndLayout : ISettingsTab
             }
             ImGuiUtil.HelpMarker(HellionStrings.Settings_ThemeAndLayout_WindowOpacity_Description);
 
+            // UI-12: inactive-window opacity, same 50-100% range and clamp.
+            var inactiveOpacityPercent = Mutable.WindowOpacityInactive * 100f;
+            if (
+                ImGuiUtil.DragFloatVertical(
+                    HellionStrings.Settings_ThemeAndLayout_WindowOpacityInactive_Name,
+                    ref inactiveOpacityPercent,
+                    .25f,
+                    50f,
+                    100f,
+                    $"{inactiveOpacityPercent:N0}%%",
+                    ImGuiSliderFlags.AlwaysClamp
+                )
+            )
+            {
+                Mutable.WindowOpacityInactive = inactiveOpacityPercent / 100f;
+            }
+            ImGuiUtil.HelpMarker(HellionStrings.Settings_ThemeAndLayout_WindowOpacityInactive_Description);
+
             ImGui.Spacing();
             ImGui.Separator();
             ImGui.Spacing();