From 104b0aef9928ff3131038bde5304b49295d67a26 Mon Sep 17 00:00:00 2001
From: AJ Isaacs <ajisaacs27@gmail.com>
Date: Mon, 12 Oct 2026 23:08:32 -0400
Subject: [PATCH] fix(clamps): retain complete groups ahead of partial fill
 tails

---
 .../Jobs/ClampZoneInteractiveFillTests.cs     | 20 +++++++++++++++++++
 .../Jobs/Placement/PlateFillService.cs        |  6 ++++--
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/OpenNest.Engine.Tests/Jobs/ClampZoneInteractiveFillTests.cs b/OpenNest.Engine.Tests/Jobs/ClampZoneInteractiveFillTests.cs
index 7430b0f..e42b36a 100644
--- a/OpenNest.Engine.Tests/Jobs/ClampZoneInteractiveFillTests.cs
+++ b/OpenNest.Engine.Tests/Jobs/ClampZoneInteractiveFillTests.cs
@@ -127,6 +127,26 @@ public class ClampZoneInteractiveFillTests
         AssertClearOfClamps(result, new Box(15, 0, 1.5, 0.625));
     }
 
+    [Fact]
+    public void FillGroupRetainsCompleteCopiesAheadOfAPartialTrailingCopy()
+    {
+        var drawing = RectDrawing(2, 2);
+        var group = new List<Part>
+        {
+            new(drawing, new Vector(0, 0)), new(drawing, new Vector(3, 0)),
+        };
+        var plate = new Plate(new Size(20, 20)) { PartSpacing = 0 };
+        var area = new Box(0, 0, 8, 2);
+        var original = PlateFillService.FillGroup("Fill", plate, group, area,
+            null, CancellationToken.None);
+        Assert.True(original.Count > group.Count && original.Count % group.Count != 0,
+            $"expected a full copy and partial tail, got {original.Count}");
+        plate.ClampZones.Add(new ClampZone("remote", new Box(18, 0, 1, 0.5)));
+        var clamped = PlateFillService.FillGroup("Fill", plate, group, area,
+            null, CancellationToken.None);
+        Assert.Equal(group.Count, clamped.Count);
+    }
+
     [Fact]
     public void FillGroupRejectsTheWholeCopyWhenOnlyOneMemberTouchesAClamp()
     {
diff --git a/OpenNest.Engine/Jobs/Placement/PlateFillService.cs b/OpenNest.Engine/Jobs/Placement/PlateFillService.cs
index 5a11871..b75e65e 100644
--- a/OpenNest.Engine/Jobs/Placement/PlateFillService.cs
+++ b/OpenNest.Engine/Jobs/Placement/PlateFillService.cs
@@ -203,9 +203,11 @@ public static class PlateFillService
         Box area, List<NestItem> demand = null, List<Part> previouslyPlaced = null,
         int groupSize = 1)
     {
-        if (proposed == null || groupSize < 1 || proposed.Count % groupSize != 0)
+        if (proposed == null || groupSize < 1)
             return;
-        for (var start = 0; start < proposed.Count; start += groupSize)
+        // FillLinear may append an incomplete trailing tile. Keep complete copies
+        // ahead of it, but never expose the incomplete tail as a group.
+        for (var start = 0; start + groupSize <= proposed.Count; start += groupSize)
         {
             // A group is one proposal: never return just its frame or just its
             // insert when a member collides with a clamp or occupied material.