Fix z-fighting between feature previews and the bodies in the Design tab

The bodies now always draw over the preview on every face a feature leaves unchanged, for
all features. Since a hole's cut sits inside the body, the Hole preview now hides the
bodies and shows the result alone, as Fillet/Chamfer and Draft do.

docs/HLSD/design-tab.md

@@ -189,6 +189,15 @@ degenerate edges are left out (`GeometryEngine::display_edges`), and the polylin
 once per shape, keyed by its `TShape`, because a recompute that leaves a body unchanged is the
 common case.
 
+While a feature card is open, its preview ghost is the whole model the candidate would produce,
+drawn translucent over the bodies, so every face the feature leaves alone is in both at the same
+depth. The ghost is drawn with a depth bias that pushes it back (`GLVolume::depth_bias`), so on a shared face
+the body always wins instead of the two copies z-fighting, and the ghost shows only where the
+result reaches past the bodies. Material a feature removes lies inside the old solid and would not
+show at all, so the tools whose result mostly coincides with the body — Fillet/Chamfer, Draft,
+Hole and the Mate hover — hide the bodies once the preview is valid and draw the result alone,
+opaque.
+
 ## Showing what is selected
 
 A selection is drawn on the faces it names, never as a tint over the body: a translucent

src/slic3r/GUI/3DScene.cpp

@@ -310,6 +310,7 @@ GLVolume::GLVolume(float r, float g, float b, float a)
     , force_native_color(false)
     , force_neutral_color(false)
     , force_sinking_contours(false)
+    , depth_bias(false)
     , picking(false)
     , tverts_range(0, size_t(-1))
 {
@@ -1332,11 +1333,17 @@ void GLVolumeCollection::render(GLVolumeCollection::ERenderType       type,
         shader->set_uniform("projection_matrix", projection_matrix);
         const Matrix3d view_normal_matrix = view_matrix.matrix().block(0, 0, 3, 3) * model_matrix.matrix().block(0, 0, 3, 3).inverse().transpose();
         shader->set_uniform("view_normal_matrix", view_normal_matrix);
+        if (volume.first->depth_bias) {
+            glsafe(::glEnable(GL_POLYGON_OFFSET_FILL));
+            glsafe(::glPolygonOffset(1.0f, 1.0f));
+        }
 		//BBS: add outline related logic
         if (volume.first->selected && shader_can_outline && GUI::wxGetApp().show_outline())
             volume.first->render_with_outline(cnv_size);
         else
             volume.first->render();
+        if (volume.first->depth_bias)
+            glsafe(::glDisable(GL_POLYGON_OFFSET_FILL));
 
 #if ENABLE_ENVIRONMENT_MAP
         if (use_environment_texture)

src/slic3r/GUI/3DScene.hpp

@@ -232,6 +232,10 @@ public:
         bool                force_neutral_color : 1;
         // Whether or not to force rendering of sinking contours
         bool                force_sinking_contours : 1;
+        // Orca: draw this volume with a positive depth bias (glPolygonOffset, pushed away from the
+        // camera), so on a surface it shares with another volume the other volume wins the depth
+        // test instead of z-fighting it
+        bool                depth_bias : 1;
         // Is render for picking
         bool                picking : 1;
         // slice error

src/slic3r/GUI/CAD/DesignCanvas.cpp

@@ -378,10 +378,14 @@ void DesignCanvas::reload(bool keep_view)
                 v->set_color(c);
             }
         } else if (obj_idx == 1) {
-            // The ghost is normally a faint blue overlay on the visible body. In preview-only
-            // mode it IS the result (base bodies hidden), so render it opaque so it reads as a
-            // finished solid rather than a see-through hint.
+            // The ghost is the whole resulting model, normally drawn as a faint blue overlay on
+            // the visible bodies. Every face the feature leaves alone is in both, at the same
+            // depth, so the ghost is drawn with a depth bias: the bodies win on those faces instead
+            // of the two copies z-fighting, and the ghost shows only where the result reaches past
+            // the bodies. In preview-only mode it IS the result (base bodies hidden), so render it
+            // opaque so it reads as a finished solid rather than a see-through hint.
             v->set_color(m_body_hidden ? ColorRGBA(0.40f, 0.82f, 1.0f, 1.0f) : ghost);
+            v->depth_bias = true;
         }
     }
 

src/slic3r/GUI/CAD/DesignPanel.cpp

@@ -11185,10 +11185,12 @@ void DesignPanel::refresh_preview()
     // Confirm so the user sees the gate before clicking; the red status says why.
     m_candidate_ok = ok;
     update_confirm_button();
-    // Fillet/Chamfer/Draft: once the target edge/face yields a valid result, show ONLY the
+    // Fillet/Chamfer/Draft/Hole: once the target edge/face yields a valid result, show ONLY the
     // preview (hide the base bodies) so the user sees the finished shape, not the old solid
-    // doubled with the ghost. Before a valid pick the body stays visible so it can be picked.
-    m_viewport->set_body_hidden((m_active == Tool::Dressup || m_active == Tool::Draft) && ok);
+    // doubled with the ghost. A hole's cut lies inside the old solid, so without this the body
+    // would hide it entirely. Before a valid pick the body stays visible so it can be picked.
+    m_viewport->set_body_hidden((m_active == Tool::Dressup || m_active == Tool::Draft ||
+                                 m_active == Tool::Hole) && ok);
     m_status->Refresh();
 
     // Refresh the in-canvas Extrude depth arrow (self-gates: only while the Extrude card is open).