diff --git a/src/AcDream.App/UI/RetailWindowManager.cs b/src/AcDream.App/UI/RetailWindowManager.cs
index 3a3640476..418e7f9da 100644
--- a/src/AcDream.App/UI/RetailWindowManager.cs
+++ b/src/AcDream.App/UI/RetailWindowManager.cs
@@ -111,6 +111,45 @@ public sealed class RetailWindowManager : IDisposable
         return handle;
     }
 
+    /// <summary>
+    /// Derives a stable authored-geometry revision from a window's own
+    /// authored extent (width, height, min width, min height, resizable), so
+    /// a plugin window's <see cref="Register"/> call can invalidate an
+    /// obsolete saved size across an authored-size change WITHOUT the plugin
+    /// author remembering to bump an explicit revision literal the way
+    /// built-in retail-imported windows do (#490 part 2 — MossTank shipped
+    /// 856x236 -> 984x271 and every stored layout stayed at 856x236 forever).
+    /// Deliberately NOT <see cref="HashCode"/>: that type reseeds its
+    /// internal state once per process specifically to defeat hash-flooding
+    /// attacks, so the SAME geometry would hash to a DIFFERENT value on
+    /// every relaunch — every login would look like a fresh authored-geometry
+    /// revision and reset every plugin window's saved size, every time. This
+    /// instead combines the exact IEEE-754 bit patterns with a fixed FNV-1a-
+    /// style multiplier, which is stable across processes, machines, and
+    /// .NET versions. <see cref="RetailWindowLayoutPersistence.MigrateAuthoredGeometry"/>
+    /// compares revisions for INEQUALITY, not ordering — a hash is not a
+    /// counter, so "authored size changed" means "the value differs",
+    /// whichever direction it moved. The sign bit is masked off the result:
+    /// <see cref="Register"/> clamps a negative <c>authoredGeometryRevision</c>
+    /// up to 0 (its "no explicit revision" sentinel), and a hash landing
+    /// there would be indistinguishable from an old, pre-hash saved layout
+    /// that never had a revision at all.
+    /// </summary>
+    public static int ComputeAuthoredGeometryRevision(
+        float width, float height, float minWidth, float minHeight, bool resizable)
+    {
+        unchecked
+        {
+            int hash = 17;
+            hash = (hash * 31) + BitConverter.SingleToInt32Bits(width);
+            hash = (hash * 31) + BitConverter.SingleToInt32Bits(height);
+            hash = (hash * 31) + BitConverter.SingleToInt32Bits(minWidth);
+            hash = (hash * 31) + BitConverter.SingleToInt32Bits(minHeight);
+            hash = (hash * 31) + (resizable ? 1 : 0);
+            return hash & 0x7FFFFFFF;
+        }
+    }
+
     public bool TryGet(string name, out RetailWindowHandle handle)
         => _byName.TryGetValue(name, out handle!);
 
diff --git a/tests/AcDream.App.Tests/UI/RetailWindowManagerTests.cs b/tests/AcDream.App.Tests/UI/RetailWindowManagerTests.cs
index 9a245023c..8172897d8 100644
--- a/tests/AcDream.App.Tests/UI/RetailWindowManagerTests.cs
+++ b/tests/AcDream.App.Tests/UI/RetailWindowManagerTests.cs
@@ -291,6 +291,48 @@ public sealed class RetailWindowManagerTests
         Assert.Equal(0, resized);
     }
 
+    // ── #490 part 2: plugin windows derive their authored-geometry revision
+    // from the authored geometry itself, so a plugin author who ships a new
+    // panel size does not also have to remember to bump an explicit revision
+    // literal the way built-in retail-imported windows do. ────────────────
+
+    [Fact]
+    public void ComputeAuthoredGeometryRevision_SameGeometry_IsStable()
+    {
+        int a = RetailWindowManager.ComputeAuthoredGeometryRevision(856f, 236f, 400f, 150f, true);
+        int b = RetailWindowManager.ComputeAuthoredGeometryRevision(856f, 236f, 400f, 150f, true);
+
+        Assert.Equal(a, b);
+    }
+
+    [Theory]
+    [InlineData(984f, 236f, 400f, 150f, true)]   // width changed (the #490 MossTank case)
+    [InlineData(856f, 271f, 400f, 150f, true)]   // height changed
+    [InlineData(856f, 236f, 420f, 150f, true)]   // minw changed
+    [InlineData(856f, 236f, 400f, 160f, true)]   // minh changed
+    [InlineData(856f, 236f, 400f, 150f, false)]  // resizable changed
+    public void ComputeAuthoredGeometryRevision_AnyFieldDiffers_ChangesTheValue(
+        float width, float height, float minWidth, float minHeight, bool resizable)
+    {
+        int baseline = RetailWindowManager.ComputeAuthoredGeometryRevision(856f, 236f, 400f, 150f, true);
+        int changed = RetailWindowManager.ComputeAuthoredGeometryRevision(
+            width, height, minWidth, minHeight, resizable);
+
+        Assert.NotEqual(baseline, changed);
+    }
+
+    [Fact]
+    public void ComputeAuthoredGeometryRevision_IsNeverNegative()
+    {
+        // Register clamps a negative authoredGeometryRevision up to 0 (its
+        // "no explicit revision" sentinel) — a hash landing there would be
+        // indistinguishable from an old, pre-hash saved layout that never
+        // had a revision at all, so the function must never produce one.
+        Assert.True(RetailWindowManager.ComputeAuthoredGeometryRevision(856f, 236f, 400f, 150f, true) >= 0);
+        Assert.True(RetailWindowManager.ComputeAuthoredGeometryRevision(0f, 0f, 0f, 0f, false) >= 0);
+        Assert.True(RetailWindowManager.ComputeAuthoredGeometryRevision(-1f, -1f, -1f, -1f, true) >= 0);
+    }
+
     private sealed class RecordingController : IRetainedPanelController
     {
         public int ShownCount { get; private set; }