fix: BoundingArea.Intersects to handle coplanar/flat area overlap

Author: mrdav30

FixedMathSharp.sln

@@ -9,6 +9,7 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "FixedMathSharp.Tests", "tes
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution Items", "{882324AE-67F3-4F5B-8485-6EF324A15CBF}"
 	ProjectSection(SolutionItems) = preProject
+		CONTRIBUTING.md = CONTRIBUTING.md
 		LICENSE.md = LICENSE.md
 		README.md = README.md
 	EndProjectSection

src/FixedMathSharp/Bounds/BoundingArea.cs

@@ -183,10 +183,24 @@ public bool Intersects(IBound other)
                         if (Contains(other.Min) && Contains(other.Max))
                             return true;  // Full containment
 
-                        // General intersection logic (allowing for overlap)
-                        return !(Max.x <= other.Min.x || Min.x >= other.Max.x ||
-                                 Max.y <= other.Min.y || Min.y >= other.Max.y ||
-                                 Max.z <= other.Min.z || Min.z >= other.Max.z);
+                        // Determine which axis is "flat" (thickness zero)
+                        bool flatX = Min.x == Max.x && other.Min.x == other.Max.x;
+                        bool flatY = Min.y == Max.y && other.Min.y == other.Max.y;
+                        bool flatZ = Min.z == Max.z && other.Min.z == other.Max.z;
+
+                        if (flatZ) // Rectangle in XY
+                            return !(Max.x < other.Min.x || Min.x > other.Max.x ||
+                                     Max.y < other.Min.y || Min.y > other.Max.y);
+                        else if (flatY) // Rectangle in XZ
+                            return !(Max.x < other.Min.x || Min.x > other.Max.x ||
+                                     Max.z < other.Min.z || Min.z > other.Max.z);
+                        else if (flatX) // Rectangle in YZ
+                            return !(Max.y < other.Min.y || Min.y > other.Max.y ||
+                                     Max.z < other.Min.z || Min.z > other.Max.z);
+                        else // fallback to 3D volume logic
+                            return !(Max.x < other.Min.x || Min.x > other.Max.x ||
+                                     Max.y < other.Min.y || Min.y > other.Max.y ||
+                                     Max.z < other.Min.z || Min.z > other.Max.z);
                     }
                 case BoundingSphere sphere:
                     // Find the closest point on the area to the sphere's center

tests/FixedMathSharp.Tests/Bounds/BoundingArea.Tests.cs

@@ -88,6 +88,10 @@ public void Intersects_WithOverlappingArea_ReturnsTrue()
             var area2 = new BoundingArea(new Vector3d(3, 3, 3), new Vector3d(6, 6, 6));
 
             Assert.True(area1.Intersects(area2));
+
+            var area3 = new BoundingArea(new Vector3d(-2, -2, 0), new Vector3d(2, 2, 0));
+            var area4 = new BoundingArea(new Vector3d(-1, -1, 0), new Vector3d(3, 3, 0));
+            Assert.True(area3.Intersects(area4));
         }
 
         [Fact]

tests/FixedMathSharp.Tests/Bounds/BoundingBox.Tests.cs

@@ -74,6 +74,10 @@ public void Intersects_WithOverlappingBox_ReturnsTrue()
             var box2 = new BoundingBox(new Vector3d(1, 1, 1), new Vector3d(4, 4, 4));
 
             Assert.True(box1.Intersects(box2));
+
+            var area3 = new BoundingBox(new Vector3d(-2, -2, 0), new Vector3d(2, 2, 0));
+            var area4 = new BoundingBox(new Vector3d(-1, -1, 0), new Vector3d(3, 3, 0));
+            Assert.False(area3.Intersects(area4));
         }
 
         [Fact]