diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index f34fb77..bdf88d6 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -8796,7 +8796,12 @@ public sealed class GameWindow : IDisposable
         int nz = 0;
         foreach (var v in copy) if (v > 0) nz++;
         if (nz == 0) return 0;
-        return copy[copy.Length - nz / 2];
+        // Sorted ascending: zero-padding at the front, samples at the back.
+        // Median of nz samples is the middle of the last nz entries; using
+        // (nz - 1) / 2 from the end keeps the offset >= 0 for all nz >= 1
+        // (the original nz / 2 form underflowed to copy.Length on first
+        // diag-flush when only 1 sample had been recorded).
+        return copy[copy.Length - 1 - (nz - 1) / 2];
     }
 
     private static long TerrainDiagPercentile95Micros(long[] samples)
@@ -8806,8 +8811,10 @@ public sealed class GameWindow : IDisposable
         int nz = 0;
         foreach (var v in copy) if (v > 0) nz++;
         if (nz == 0) return 0;
-        int idx = copy.Length - 1 - (int)(nz * 0.05);
-        return copy[idx];
+        // 95th percentile = upper end of the sorted samples; clamp the
+        // offset to stay inside the populated tail when nz < 20.
+        int offset = (int)((nz - 1) * 0.05);
+        return copy[copy.Length - 1 - offset];
     }
 
     private void OnClosing()