diff --git a/lib/Slic3r/Print/Object.pm b/lib/Slic3r/Print/Object.pm
index 791731543..438c4b1ef 100644
--- a/lib/Slic3r/Print/Object.pm
+++ b/lib/Slic3r/Print/Object.pm
@@ -965,7 +965,11 @@ sub discover_horizontal_shells {
                 # solution so far. Growing the external slices by EXTERNAL_INFILL_MARGIN will put
                 # too much solid infill inside nearly-vertical slopes.
                 my $solid = [
+                    # Surfaces including the area of perimeters. Everything, that is visible from the top / bottom
+                    # (not covered by a layer above / below).
+                    # This does not contain the areas covered by perimeters!
                     (map $_->p, @{$layerm->slices->filter_by_type($type)}),
+                    # Infill areas (slices without the perimeters).
                     (map $_->p, @{$layerm->fill_surfaces->filter_by_type($type)}),
                 ];
                 next if !@$solid;
@@ -1011,20 +1015,28 @@ sub discover_horizontal_shells {
                         # and it's not wanted in a hollow print even if it would make sense when
                         # obeying the solid shell count option strictly (DWIM!)
                         my $margin = $neighbor_layerm->flow(FLOW_ROLE_EXTERNAL_PERIMETER)->scaled_width;
+                        my $regularized = offset2($new_internal_solid, -$margin, +$margin, CLIPPER_OFFSET_SCALE, JT_MITER, 5);
                         my $too_narrow = diff(
                             $new_internal_solid,
-                            offset2($new_internal_solid, -$margin, +$margin, CLIPPER_OFFSET_SCALE, JT_MITER, 5),
+                            $regularized,
                             1,
                         );
-                        $new_internal_solid = $solid = diff(
+                        # Trim the regularized region by the original region.
+                        $new_internal_solid = $solid = intersection(
                             $new_internal_solid,
-                            $too_narrow,
+                            $regularized,
                         ) if @$too_narrow;
                     }
                     
                     # make sure the new internal solid is wide enough, as it might get collapsed
                     # when spacing is added in Fill.pm
-                    if (0) {
+                    if ($layerm->region->config->ensure_vertical_shell_thickness) {
+                        # The possible thin sickles of top / bottom surfaces on steeply sloping surfaces touch
+                        # the projections of top / bottom perimeters, therefore they will be sufficiently inflated by
+                        # merging them with the projections of the top / bottom perimeters.
+                    } else {
+                        #FIXME Vojtech: Disable this and you will be sorry.
+                        # https://github.com/prusa3d/Slic3r/issues/26 bottom
                         my $margin = 3 * $layerm->flow(FLOW_ROLE_SOLID_INFILL)->scaled_width; # require at least this size
                         # we use a higher miterLimit here to handle areas with acute angles
                         # in those cases, the default miterLimit would cut the corner and we'd
diff --git a/xs/src/libslic3r/LayerRegion.cpp b/xs/src/libslic3r/LayerRegion.cpp
index f25ad2dc0..23a1bd9b0 100644
--- a/xs/src/libslic3r/LayerRegion.cpp
+++ b/xs/src/libslic3r/LayerRegion.cpp
@@ -377,6 +377,7 @@ LayerRegion::prepare_fill_surfaces()
 
 #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
     export_region_slices_to_svg_debug("2_prepare_fill_surfaces");
+    export_region_fill_surfaces_to_svg_debug("2_prepare_fill_surfaces");
 #endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
 }
 
@@ -426,8 +427,10 @@ void LayerRegion::export_region_fill_surfaces_to_svg(const char *path)
 
     SVG svg(path, bbox);
     const float transparency = 0.5f;
-    for (Surfaces::const_iterator surface = this->fill_surfaces.surfaces.begin(); surface != this->fill_surfaces.surfaces.end(); ++surface)
+    for (Surfaces::const_iterator surface = this->fill_surfaces.surfaces.begin(); surface != this->fill_surfaces.surfaces.end(); ++surface) {
         svg.draw(surface->expolygon, surface_type_to_color_name(surface->surface_type), transparency);
+        svg.draw_outline(surface->expolygon, "black", "blue", scale_(0.05)); 
+    }
     export_surface_type_legend_to_svg(svg, legend_pos);
     svg.Close();
 }
diff --git a/xs/src/libslic3r/PrintObject.cpp b/xs/src/libslic3r/PrintObject.cpp
index 8fbbf8c3e..34f6fbbb8 100644
--- a/xs/src/libslic3r/PrintObject.cpp
+++ b/xs/src/libslic3r/PrintObject.cpp
@@ -370,6 +370,7 @@ PrintObject::discover_vertical_shells()
                 for (int n = (int)idx_layer - layerm->region()->config.bottom_solid_layers + 1; n < (int)idx_layer + layerm->region()->config.top_solid_layers; ++ n)
                     if (n >= 0 && n < (int)this->layers.size())
                         polygons_append(shell, this->layers[n]->perimeter_expolygons.expolygons);
+                //FIXME Add the top / bottom layerm->slices to the mix!
 #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                 {
                     static size_t idx = 0;