Fixed the issue that an object disappeared in a multi-material painting gizmo when the object was almost completely sunk under the bed.

Object disappearance was caused by calling glPolygonOffset for rendered triangles in the multi-material painting gizmo to resolve z-fighting between painted triangles and contours around selected areas using seed/bucket fill.

resources/shaders/mm_contour.fs

@@ -1,6 +1,11 @@
 #version 110
 
+const float EPSILON = 0.0001;
+
 void main()
 {
     gl_FragColor = vec4(1.0, 1.0, 1.0, 1.0);
+    // Values inside depth buffer for fragments of the contour of a selected area are offset
+    // by small epsilon to solve z-fighting between painted triangles and contour lines.
+    gl_FragDepth = gl_FragCoord.z - EPSILON;
 }

src/slic3r/GUI/Gizmos/GLGizmoMmuSegmentation.cpp

@@ -630,7 +630,11 @@ void TriangleSelectorMmGui::render(ImGuiWrapper *imgui)
 
         auto *contour_shader = wxGetApp().get_shader("mm_contour");
         contour_shader->start_using();
+
+        glsafe(::glDepthFunc(GL_LEQUAL));
         m_gizmo_scene.render_contour();
+        glsafe(::glDepthFunc(GL_LESS));
+
         contour_shader->stop_using();
     }
 
@@ -725,10 +729,6 @@ void GLMmSegmentationGizmo3DScene::render(size_t triangle_indices_idx) const
     assert(this->vertices_VBO_id != 0);
     assert(this->triangle_indices_VBO_ids[triangle_indices_idx] != 0);
 
-    ScopeGuard offset_fill_guard([]() { glsafe(::glDisable(GL_POLYGON_OFFSET_FILL)); });
-    glsafe(::glEnable(GL_POLYGON_OFFSET_FILL));
-    glsafe(::glPolygonOffset(5.0, 5.0));
-
     glsafe(::glBindBuffer(GL_ARRAY_BUFFER, this->vertices_VBO_id));
     glsafe(::glVertexPointer(3, GL_FLOAT, 3 * sizeof(float), (const void*)(0 * sizeof(float))));