diff --git a/t/bridges.t b/t/bridges.t
index 95db67daf..677115b84 100644
--- a/t/bridges.t
+++ b/t/bridges.t
@@ -1,4 +1,4 @@
-use Test::More tests => 18;
+use Test::More tests => 20;
 use strict;
 use warnings;
 
@@ -99,6 +99,24 @@ use Slic3r::Test;
         'correct bridge angle for rectangle';
 }
 
+{
+    # GH #3929: This test case checks that narrow gaps in lower slices don't prevent correct
+    # direction detection.
+    my $bridge = Slic3r::ExPolygon->new(
+        Slic3r::Polygon->new([10099996,45867519],[3762370,45867519],[3762370,2132479],[10099996,2132479]),
+    );
+    my $lower = [
+        Slic3r::ExPolygon->new(
+            Slic3r::Polygon->new([13534103,210089],[13629884,235753],[14249999,401901],[14269611,421510],[14272931,424830],[14287518,439411],[14484206,636101],[15348099,1500000],[15360812,1547449],[15365467,1564815],[15388623,1651235],[15391897,1663454],[15393088,1667906],[15399044,1690134],[15457593,1908648],[15750000,2999999],[15750000,45000000],[15742825,45026783],[15741540,45031580],[15735900,45052628],[15663980,45321047],[15348099,46500000],[15151410,46696691],[14287518,47560587],[14267907,47580196],[14264587,47583515],[14249999,47598100],[14211041,47608539],[14204785,47610215],[14176024,47617916],[14105602,47636784],[14097768,47638884],[14048000,47652220],[13871472,47699515],[12750000,48000000],[10446106,48000000],[10446124,47990347],[10446124,9652],[10446106,0],[12750000,0]),
+            Slic3r::Polygon->new([10251886,5013],[10251886,47994988],[10251907,48000000],[10100006,48000000],[10100006,0],[10251907,0]),
+            Slic3r::Polygon->new([3762360,17017],[3762360,47982984],[3762397,48000000],[1249999,48000000],[536029,47808700],[456599,47787419],[73471,47684764],[0,47665076],[0,23124327],[119299,22907322],[159278,22834601],[196290,22690451],[239412,22522516],[303787,22271780],[639274,20965103],[639274,19034896],[616959,18947983],[607651,18911729],[559146,18722807],[494769,18472073],[159278,17165397],[38931,16946491],[0,16875676],[0,334922],[128529,300484],[1250000,0],[3762397,0]),
+        ),
+    ];
+    
+    ok check_angle($lower, $bridge, 0, undef, $bridge->area, 500000),
+        'correct bridge angle when lower slices have narrow gap';
+}
+
 sub check_angle {
     my ($lower, $bridge, $expected, $tolerance, $expected_coverage, $extrusion_width) = @_;
     
@@ -121,9 +139,9 @@ sub check_angle {
     # our epsilon is equal to the steps used by the bridge detection algorithm
     ###use XXX; YYY [ rad2deg($result), $expected ];
     # returned value must be non-negative, check for that too
-    my $delta=rad2deg($result) - $expected;
-    $delta-=180 if $delta>=180 - epsilon;
-    return defined $result && $result>=0 && abs($delta) < $tolerance;
+    my $delta = rad2deg($result) - $expected;
+    $delta -= 180 if $delta >= 180 - epsilon;
+    return defined $result && $result >= 0 && abs($delta) < $tolerance;
 }
 
 {
diff --git a/xs/src/libslic3r/BridgeDetector.cpp b/xs/src/libslic3r/BridgeDetector.cpp
index 33c239d6f..255b78a2a 100644
--- a/xs/src/libslic3r/BridgeDetector.cpp
+++ b/xs/src/libslic3r/BridgeDetector.cpp
@@ -7,13 +7,23 @@ namespace Slic3r {
 
 BridgeDetector::BridgeDetector(const ExPolygon &_expolygon, const ExPolygonCollection &_lower_slices,
     coord_t _extrusion_width)
-    : expolygon(_expolygon), lower_slices(_lower_slices), extrusion_width(_extrusion_width),
+    : expolygon(_expolygon), extrusion_width(_extrusion_width),
         resolution(PI/36.0), angle(-1)
 {
     /*  outset our bridge by an arbitrary amout; we'll use this outer margin
         for detecting anchors */
     Polygons grown = offset(this->expolygon, this->extrusion_width);
     
+    // remove narrow gaps from lower slices
+    // (this is only needed as long as we use clipped test lines for angle detection
+    // and we check their endpoints: when endpoint fall in the gap we'd get false
+    // negatives)
+    this->lower_slices.expolygons = offset2_ex(
+        _lower_slices,
+        +this->extrusion_width/2,
+        -this->extrusion_width/2
+    );
+    
     // detect what edges lie on lower slices by turning bridge contour and holes
     // into polylines and then clipping them with each lower slice's contour
     this->_edges = intersection_pl(grown, this->lower_slices.contours());
@@ -34,6 +44,10 @@ BridgeDetector::BridgeDetector(const ExPolygon &_expolygon, const ExPolygonColle
         svg.draw(this->_anchors, "yellow");
         svg.draw(this->_edges, "black", scale_(0.2));
         svg.Close();
+        
+        std::cout << "expolygon: " << this->expolygon.dump_perl() << std::endl;
+        for (const ExPolygon &e : this->lower_slices.expolygons)
+            std::cout << "lower: " << e.dump_perl() << std::endl;
     }
     #endif
 }
@@ -133,6 +147,13 @@ BridgeDetector::detect_angle()
             ));
         }
         if (candidate.coverage > 0) have_coverage = true;
+        
+        #if 0
+        std::cout << "angle = "  << Slic3r::Geometry::rad2deg(candidate.angle)
+            << "; coverage = "   << candidate.coverage
+            << "; max_length = " << candidate.max_length
+            << std::endl;
+        #endif
     }
     
     // if no direction produced coverage, then there's no bridge direction
@@ -159,12 +180,16 @@ BridgeDetector::detect_angle()
     return true;
 }
 
+Polygons
+BridgeDetector::coverage() const
+{
+    if (this->angle == -1) return Polygons();
+    return this->coverage(this->angle);
+}
+
 Polygons
 BridgeDetector::coverage(double angle) const
 {
-    if (angle == -1) angle = this->angle;
-    if (angle == -1) return Polygons();
-    
     // Clone our expolygon and rotate it so that we work with vertical lines.
     ExPolygon expolygon = this->expolygon;
     expolygon.rotate(PI/2.0 - angle, Point(0,0));
diff --git a/xs/src/libslic3r/BridgeDetector.hpp b/xs/src/libslic3r/BridgeDetector.hpp
index 84f9f3848..9abe1162a 100644
--- a/xs/src/libslic3r/BridgeDetector.hpp
+++ b/xs/src/libslic3r/BridgeDetector.hpp
@@ -23,7 +23,8 @@ public:
     
     BridgeDetector(const ExPolygon &_expolygon, const ExPolygonCollection &_lower_slices, coord_t _extrusion_width);
     bool detect_angle();
-    Polygons coverage(double angle = -1) const;
+    Polygons coverage() const;
+    Polygons coverage(double angle) const;
     Polylines unsupported_edges(double angle = -1) const;
     
 private:
diff --git a/xs/src/libslic3r/Fill/Fill.cpp b/xs/src/libslic3r/Fill/Fill.cpp
index 39c9107b9..c07851fb1 100644
--- a/xs/src/libslic3r/Fill/Fill.cpp
+++ b/xs/src/libslic3r/Fill/Fill.cpp
@@ -1,10 +1,9 @@
-#define DEBUG
-#undef NDEBUG
 #include <cassert>
 #include <math.h>
 #include <stdio.h>
 
 #include "../ClipperUtils.hpp"
+#include "../Geometry.hpp"
 #include "../Surface.hpp"
 #include "../PrintConfig.hpp"
 
@@ -96,10 +95,8 @@ Fill::_infill_direction(const Surface &surface) const
 
     if (surface.bridge_angle >= 0) {
 	    // use bridge angle
-		//FIXME Vojtech: Add a debugf?
-        // Slic3r::debugf "Filling bridge with angle %d\n", rad2deg($surface->bridge_angle);
         #ifdef SLIC3R_DEBUG
-        printf("Filling bridge with angle %f\n", surface.bridge_angle);
+        printf("Filling bridge with angle %f\n", Slic3r::Geometry::rad2deg(surface.bridge_angle));
         #endif
         out_angle = surface.bridge_angle;
     } else if (this->layer_id != size_t(-1)) {