diff --git a/src/slic3r/GUI/3DScene.cpp b/src/slic3r/GUI/3DScene.cpp
index fd3569e2ca7..50d4cfcfa6f 100644
--- a/src/slic3r/GUI/3DScene.cpp
+++ b/src/slic3r/GUI/3DScene.cpp
@@ -978,12 +978,51 @@ GLVolumeWithIdAndZList volumes_to_render(const GLVolumePtrs& volumes, GLVolumeCo
     return list;
 }
 
-int GLVolumeCollection::get_selection_support_threshold_angle(bool &enable_support) const
+// ORCA: Compute slope.normal_z for 3D overhang highlight directly from support settings.
+// If support_threshold_angle is 0, use tree fallback angle (30 deg) for tree supports,
+// and derive an equivalent angle from threshold overlap for normal supports.
+float GLVolumeCollection::get_selection_support_normal_z() const
 {
-    const DynamicPrintConfig& glb_cfg        = GUI::wxGetApp().preset_bundle->prints.get_edited_preset().config;
-    enable_support =  glb_cfg.opt_bool("enable_support");
-    int support_threshold_angle =  glb_cfg.opt_int("support_threshold_angle");
-    return  support_threshold_angle ;
+    const DynamicPrintConfig& glb_cfg  = GUI::wxGetApp().preset_bundle->prints.get_edited_preset().config;
+    const auto& full_cfg               = GUI::wxGetApp().preset_bundle->full_config();
+    const auto support_type            = glb_cfg.opt_enum<SupportType>("support_type");
+    const int  support_threshold_angle = glb_cfg.opt_int("support_threshold_angle");
+    double angle_rad;
+
+    if (support_threshold_angle > 0) {
+        // Match support generation: explicit threshold angles are treated as inclusive.
+        const int effective_support_threshold_angle = std::min(support_threshold_angle + 1, 89);
+        angle_rad = Geometry::deg2rad(static_cast<double>(effective_support_threshold_angle));
+    } else if (is_tree(support_type)) {
+        angle_rad = Geometry::deg2rad(30.0); // fallback value for tree supports
+    } else { // For normal supports, if the angle is set to 0, calculate normal_z from overlap.
+        const double layer_height        = full_cfg.opt_float("layer_height");
+        const auto*  nozzle_diameter_opt = full_cfg.option<ConfigOptionFloats>("nozzle_diameter");
+        const int    wall_filament       = full_cfg.opt_int("wall_filament");
+        const size_t nozzle_count        = nozzle_diameter_opt->values.size();
+        const size_t wall_extruder_idx   = (wall_filament > 0 && wall_filament <= static_cast<int>(nozzle_count))
+            ? static_cast<size_t>(wall_filament - 1)
+            : 0; // Invalid extruder index falls back to extruder 1.
+        
+        // Use wall extruder's nozzle diameter for better estimation of external perimeter width,
+        // which is more relevant to overhang printing than the default nozzle diameter.
+        const double nozzle_diameter = nozzle_diameter_opt->values[wall_extruder_idx];
+
+        double external_perimeter_width = full_cfg.get_abs_value("outer_wall_line_width", nozzle_diameter);
+        if (external_perimeter_width <= 0.0) {
+            external_perimeter_width = full_cfg.get_abs_value("line_width", nozzle_diameter);
+
+            if (external_perimeter_width <= 0.0)
+                external_perimeter_width = nozzle_diameter;
+        }
+
+        const double overlap_width      = full_cfg.get_abs_value("support_threshold_overlap", external_perimeter_width);
+        const double lower_layer_offset = std::max(0.0, external_perimeter_width - overlap_width);
+
+        angle_rad = lower_layer_offset <= EPSILON ? Geometry::deg2rad(89.0) : std::atan(layer_height / lower_layer_offset);
+    }
+
+    return static_cast<float>(-std::cos(std::clamp(angle_rad, 0.0, Geometry::deg2rad(89.0))));
 }
 
 //BBS: add outline drawing logic
@@ -1019,6 +1058,8 @@ void GLVolumeCollection::render(GLVolumeCollection::ERenderType       type,
     if (disable_cullface)
         glsafe(::glDisable(GL_CULL_FACE));
 
+    const float support_normal_z = get_selection_support_normal_z();
+
     for (GLVolumeWithIdAndZ& volume : to_render) {
 #if ENABLE_MODIFIERS_ALWAYS_TRANSPARENT
         if (type == ERenderType::Transparent) {
@@ -1075,16 +1116,11 @@ void GLVolumeCollection::render(GLVolumeCollection::ERenderType       type,
             //use -1 ad a invalid type
             shader->set_uniform("print_volume.type", -1);
         }
-        
-        bool  enable_support;
-        int   support_threshold_angle = get_selection_support_threshold_angle(enable_support);
-    
-        float normal_z  = -::cos(Geometry::deg2rad((float) support_threshold_angle));
   
         shader->set_uniform("volume_world_matrix", volume.first->world_matrix());
         shader->set_uniform("slope.actived", m_slope.isGlobalActive && !volume.first->is_modifier && !volume.first->is_wipe_tower);
         shader->set_uniform("slope.volume_world_normal_matrix", static_cast<Matrix3f>(volume.first->world_matrix().matrix().block(0, 0, 3, 3).inverse().transpose().cast<float>()));
-        shader->set_uniform("slope.normal_z", normal_z);
+        shader->set_uniform("slope.normal_z", support_normal_z);
 
 #if ENABLE_ENVIRONMENT_MAP
         unsigned int environment_texture_id = GUI::wxGetApp().plater()->get_environment_texture_id();
diff --git a/src/slic3r/GUI/3DScene.hpp b/src/slic3r/GUI/3DScene.hpp
index b12d048aa9a..76d10620d96 100644
--- a/src/slic3r/GUI/3DScene.hpp
+++ b/src/slic3r/GUI/3DScene.hpp
@@ -488,7 +488,7 @@ public:
     GLVolume* new_toolpath_volume(const ColorRGBA& rgba);
     GLVolume* new_nontoolpath_volume(const ColorRGBA& rgba);
 
-    int get_selection_support_threshold_angle(bool&) const;
+    float get_selection_support_normal_z() const;
     // Render the volumes by OpenGL.
     //BBS: add outline drawing logic
     void render(ERenderType                           type,