extent also ensures 3Dness

src/meshTools/octree/treeBoundBox.H

@@ -314,6 +314,7 @@ public:
 
             //- Return slightly wider bounding box
             //  Extends all dimensions with s*span*Random::scalar01()
+            //  and guarantees in any direction s*mag(span) minimum width
             inline treeBoundBox extend(Random&, const scalar s) const;
 
     // Friend Operators

src/meshTools/octree/treeBoundBoxI.H

@@ -437,7 +437,15 @@ inline treeBoundBox treeBoundBox::extend(Random& rndGen, const scalar s) const
 {
     treeBoundBox bb(*this);
 
-    const vector span(bb.max() - bb.min());
+    vector span(bb.max() - bb.min());
+
+    // Make 3D
+    scalar magSpan = Foam::mag(span);
+
+    for (direction dir = 0; dir < vector::nComponents; dir++)
+    {
+        span[dir] = Foam::max(s*magSpan, span[dir]);
+    }
 
     bb.min() -= cmptMultiply(s*rndGen.vector01(), span);
     bb.max() += cmptMultiply(s*rndGen.vector01(), span);