diff --git a/src/db/db/dbAsIfFlatRegion.cc b/src/db/db/dbAsIfFlatRegion.cc
index a13e49823..e6cd117c1 100644
--- a/src/db/db/dbAsIfFlatRegion.cc
+++ b/src/db/db/dbAsIfFlatRegion.cc
@@ -764,7 +764,7 @@ AsIfFlatRegion::sized (coord_type dx, coord_type dy, unsigned int mode) const
     db::Box b = bbox ().enlarged (db::Vector (dx, dy));
     return region_from_box (b);
 
-  } else if (! merged_semantics ()) {
+  } else if (! merged_semantics () || is_merged ()) {
 
     //  Generic case
     std::auto_ptr<FlatRegion> new_region (new FlatRegion (false /*output isn't merged*/));
@@ -801,7 +801,7 @@ AsIfFlatRegion::sized (coord_type dx, coord_type dy, unsigned int mode) const
     db::ShapeGenerator pc (new_region->raw_polygons (), true /*clear*/);
     db::PolygonGenerator pg2 (pc, false /*don't resolve holes*/, true /*min. coherence*/);
     db::SizingPolygonFilter siz (pg2, dx, dy, mode);
-    db::PolygonGenerator pg (siz, false /*don't resolve holes*/, false /*min. coherence*/);
+    db::PolygonGenerator pg (siz, false /*don't resolve holes*/, min_coherence () /*min. coherence*/);
     db::BooleanOp op (db::BooleanOp::Or);
     ep.process (pg, op);
 
diff --git a/src/db/db/dbDeepRegion.cc b/src/db/db/dbDeepRegion.cc
index 0ee8b7195..572e7a6ba 100644
--- a/src/db/db/dbDeepRegion.cc
+++ b/src/db/db/dbDeepRegion.cc
@@ -160,6 +160,11 @@ void DeepRegion::merged_semantics_changed ()
   //  .. nothing yet ..
 }
 
+void DeepRegion::min_coherence_changed ()
+{
+  set_is_merged (false);
+}
+
 RegionIteratorDelegate *
 DeepRegion::begin () const
 {
diff --git a/src/db/db/dbDeepRegion.h b/src/db/db/dbDeepRegion.h
index 6ede2cee0..ce554ffb9 100644
--- a/src/db/db/dbDeepRegion.h
+++ b/src/db/db/dbDeepRegion.h
@@ -220,6 +220,7 @@ public:
 
 protected:
   virtual void merged_semantics_changed ();
+  virtual void min_coherence_changed ();
   void set_is_merged (bool f);
 
 private:
diff --git a/src/db/db/dbFlatRegion.cc b/src/db/db/dbFlatRegion.cc
index 1ab7ebaa9..3df87bf77 100644
--- a/src/db/db/dbFlatRegion.cc
+++ b/src/db/db/dbFlatRegion.cc
@@ -94,6 +94,13 @@ void FlatRegion::merged_semantics_changed ()
   m_merged_polygons_valid = false;
 }
 
+void FlatRegion::min_coherence_changed ()
+{
+  m_is_merged = false;
+  m_merged_polygons.clear ();
+  m_merged_polygons_valid = false;
+}
+
 void FlatRegion::reserve (size_t n)
 {
   m_polygons.reserve (db::Polygon::tag (), n);
diff --git a/src/db/db/dbFlatRegion.h b/src/db/db/dbFlatRegion.h
index 5fe5e50ec..5f7d2d2cd 100644
--- a/src/db/db/dbFlatRegion.h
+++ b/src/db/db/dbFlatRegion.h
@@ -180,6 +180,7 @@ public:
 
 protected:
   virtual void merged_semantics_changed ();
+  virtual void min_coherence_changed ();
   virtual Box compute_bbox () const;
   void invalidate_cache ();
   void set_is_merged (bool m);
diff --git a/src/db/db/dbOriginalLayerRegion.cc b/src/db/db/dbOriginalLayerRegion.cc
index 73f9aa83f..28a79033f 100644
--- a/src/db/db/dbOriginalLayerRegion.cc
+++ b/src/db/db/dbOriginalLayerRegion.cc
@@ -151,6 +151,14 @@ OriginalLayerRegion::merged_semantics_changed ()
   m_merged_polygons_valid = false;
 }
 
+void
+OriginalLayerRegion::min_coherence_changed ()
+{
+  m_is_merged = false;
+  m_merged_polygons.clear ();
+  m_merged_polygons_valid = false;
+}
+
 RegionIteratorDelegate *
 OriginalLayerRegion::begin () const
 {
diff --git a/src/db/db/dbOriginalLayerRegion.h b/src/db/db/dbOriginalLayerRegion.h
index b99a651ec..23ce7004b 100644
--- a/src/db/db/dbOriginalLayerRegion.h
+++ b/src/db/db/dbOriginalLayerRegion.h
@@ -72,6 +72,7 @@ public:
 
 protected:
   virtual void merged_semantics_changed ();
+  virtual void min_coherence_changed ();
 
 private:
   OriginalLayerRegion &operator= (const OriginalLayerRegion &other);
diff --git a/src/db/db/dbRegionDelegate.cc b/src/db/db/dbRegionDelegate.cc
index 0e6328ce7..2ce675465 100644
--- a/src/db/db/dbRegionDelegate.cc
+++ b/src/db/db/dbRegionDelegate.cc
@@ -78,7 +78,10 @@ void RegionDelegate::set_base_verbosity (int vb)
 
 void RegionDelegate::set_min_coherence (bool f)
 {
-  m_merge_min_coherence = f;
+  if (f != m_merge_min_coherence) {
+    m_merge_min_coherence = f;
+    min_coherence_changed ();
+  }
 }
 
 void RegionDelegate::set_merged_semantics (bool f)
diff --git a/src/db/db/dbRegionDelegate.h b/src/db/db/dbRegionDelegate.h
index f5eb945d7..8fe79edb9 100644
--- a/src/db/db/dbRegionDelegate.h
+++ b/src/db/db/dbRegionDelegate.h
@@ -319,6 +319,7 @@ protected:
   }
 
   virtual void merged_semantics_changed () { }
+  virtual void min_coherence_changed () { }
 
 private:
   bool m_merged_semantics;
diff --git a/src/db/unit_tests/dbDeepRegionTests.cc b/src/db/unit_tests/dbDeepRegionTests.cc
index a23807ecb..5bdb28766 100644
--- a/src/db/unit_tests/dbDeepRegionTests.cc
+++ b/src/db/unit_tests/dbDeepRegionTests.cc
@@ -1406,3 +1406,30 @@ TEST(101_DeepFlatCollaboration)
   db::compare_layouts (_this, target, tl::testsrc () + "/testdata/algo/deep_region_au101.gds");
 }
 
+TEST(issue_277)
+{
+  db::Layout ly;
+  db::cell_index_type top_cell_index = ly.add_cell ("TOP");
+  db::Cell &top_cell = ly.cell (top_cell_index);
+  unsigned int l1 = ly.insert_layer ();
+
+  db::Shapes &s = top_cell.shapes (l1);
+  s.insert (db::Box (0, 0, 400, 400));
+  s.insert (db::Box (400, 400, 800, 800));
+
+
+  db::DeepShapeStore dss;
+
+  db::Region r (db::RecursiveShapeIterator (ly, top_cell, l1), dss);
+
+  EXPECT_EQ (r.sized (1).merged (false, 1).to_string (), "");
+
+  r.set_min_coherence (true);
+  EXPECT_EQ (r.sized (1).merged (false, 1).to_string (), "(399,399;399,401;401,401;401,399)");
+
+  r.merge ();
+  EXPECT_EQ (r.sized (1).merged (false, 1).to_string (), "(399,399;399,401;401,401;401,399)");
+
+  r.set_min_coherence (false);  //  needs to merge again
+  EXPECT_EQ (r.sized (1).merged (false, 1).to_string (), "");
+}
diff --git a/src/db/unit_tests/dbRegion.cc b/src/db/unit_tests/dbRegion.cc
index 6fb883ae1..31d939d4a 100644
--- a/src/db/unit_tests/dbRegion.cc
+++ b/src/db/unit_tests/dbRegion.cc
@@ -1397,3 +1397,21 @@ TEST(issue_228)
   EXPECT_EQ (r.selected_interacting (rr).to_string (), r.to_string ());
   EXPECT_EQ (rr.selected_interacting (r).to_string (), rr.to_string ());
 }
+
+TEST(issue_277)
+{
+  db::Region r;
+  r.insert (db::Box (0, 0, 400, 400));
+  r.insert (db::Box (400, 400, 800, 800));
+
+  EXPECT_EQ (r.sized (1).merged (false, 1).to_string (), "");
+
+  r.set_min_coherence (true);
+  EXPECT_EQ (r.sized (1).merged (false, 1).to_string (), "(399,399;399,401;401,401;401,399)");
+
+  r.merge ();
+  EXPECT_EQ (r.sized (1).merged (false, 1).to_string (), "(399,399;399,401;401,401;401,399)");
+
+  r.set_min_coherence (false);  //  needs to merge again
+  EXPECT_EQ (r.sized (1).merged (false, 1).to_string (), "");
+}