diff --git a/src/ifcviewer/ViewportCore.cpp b/src/ifcviewer/ViewportCore.cpp
index cef5199cc7..4ad8e46a88 100644
--- a/src/ifcviewer/ViewportCore.cpp
+++ b/src/ifcviewer/ViewportCore.cpp
@@ -5268,3 +5268,52 @@ void ViewportCore::render() {
         }
     }
 }
+
+// ===========================================================================
+// Section planes (#84-y)
+// ===========================================================================
+
+bool ViewportCore::addSectionPlaneAtSurface(const Eigen::Vector3f& point,
+                                            const Eigen::Vector3f& normal,
+                                            float visual_radius) {
+    if (int(section_planes_.size()) >= kMaxSectionPlanes) {
+        Log::warn() << "[wgpu section] cap reached (" << kMaxSectionPlanes
+                    << " planes)";
+        return false;
+    }
+    Eigen::Vector3f n = normal;
+    if (n.squaredNorm() < 1e-8f) return false;
+    n.normalize();
+    // Auto-flip the normal so the camera-facing half gets cut away.
+    const Eigen::Vector3f eye = orbitEye(camera_target_, camera_distance_,
+                                         camera_yaw_deg_, camera_pitch_deg_);
+    const Eigen::Vector3f eye_dir = eye - point;
+    if (n.dot(eye_dir) < 0.0f) n = -n;
+
+    SectionPlane p;
+    p.n             = n;
+    p.origin        = point;
+    p.d             = -n.dot(point);
+    p.visual_radius = (visual_radius > 0.0f) ? visual_radius : 1.0f;
+    section_planes_.push_back(p);
+    Log::info()
+        << "[wgpu section] added plane #" << section_planes_.size() - 1
+        << " origin=(" << point.x() << "," << point.y() << "," << point.z() << ")"
+        << " normal=(" << n.x() << "," << n.y() << "," << n.z() << ")";
+    host_->requestFrame();
+    return true;
+}
+
+void ViewportCore::removeSectionPlane(int index) {
+    if (index < 0 || index >= int(section_planes_.size())) return;
+    section_planes_.erase(section_planes_.begin() + index);
+    Log::info() << "[wgpu section] removed plane " << index;
+    host_->requestFrame();
+}
+
+void ViewportCore::clearSectionPlanes() {
+    if (section_planes_.empty()) return;
+    section_planes_.clear();
+    Log::info() << "[wgpu section] cleared all planes";
+    host_->requestFrame();
+}
diff --git a/src/ifcviewer/ViewportCore.h b/src/ifcviewer/ViewportCore.h
index 3df584bdfb..01755df9e2 100644
--- a/src/ifcviewer/ViewportCore.h
+++ b/src/ifcviewer/ViewportCore.h
@@ -355,6 +355,23 @@ public:
     void finalizeScreenshotCapture(WGPUBuffer capture_buffer,
                                    std::uint32_t padded_bpr);
 
+    // ---- Section planes (#84-y) -------------------------------------------
+    //
+    // Append a section plane at the supplied surface hit, with a normal
+    // auto-flipped toward the camera so the first click reveals the
+    // surface the user just clicked. `visual_radius` controls the
+    // overlay gizmo size; <= 0 falls back to 1 m. Returns false when
+    // the kMaxSectionPlanes cap is already reached.
+    bool addSectionPlaneAtSurface(const Eigen::Vector3f& point,
+                                  const Eigen::Vector3f& normal,
+                                  float visual_radius);
+
+    // Remove a single section plane by index (no-op when out of range).
+    void removeSectionPlane(int index);
+
+    // Drop every section plane. No-op when none are active.
+    void clearSectionPlanes();
+
     // ---- Render loop (#84-x) ----------------------------------------------
     //
     // Encode one frame: acquire the swapchain texture, run cull (parallel
diff --git a/src/ifcviewer/ViewportWindow.cpp b/src/ifcviewer/ViewportWindow.cpp
index 4e8264b72a..dd73d27b25 100644
--- a/src/ifcviewer/ViewportWindow.cpp
+++ b/src/ifcviewer/ViewportWindow.cpp
@@ -969,51 +969,13 @@ void ViewportWindow::toggleSectionTool() {
     if (isExposed()) requestUpdate();
 }
 
-bool ViewportWindow::addSectionPlaneAtSurface(const Eigen::Vector3f& point,
-                                                  const Eigen::Vector3f& normal,
-                                                  float visual_radius) {
-    if (int(section_planes_.size()) >= kMaxSectionPlanes) {
-        std::fprintf(stderr, "[warn] [wgpu section] cap reached (%d planes)\n",
-                     kMaxSectionPlanes);
-        return false;
-    }
-    Eigen::Vector3f n = normal;
-    if (n.squaredNorm() < 1e-8f) return false;
-    n.normalize();
-    // Auto-flip the normal so the camera-facing half gets cut away — that
-    // way the first click always reveals the surface the user just clicked.
-    const Eigen::Vector3f eye = orbitEye(camera_target_, camera_distance_,
-                                   camera_yaw_deg_, camera_pitch_deg_);
-    const Eigen::Vector3f eye_dir = eye - point;
-    if (n.dot(eye_dir) < 0.0f) n = -n;
-
-    SectionPlane p;
-    p.n             = n;
-    p.origin        = point;
-    p.d             = -n.dot(point);
-    p.visual_radius = (visual_radius > 0.0f) ? visual_radius : 1.0f;
-    section_planes_.push_back(p);
-    Log::info().noquote().nospace()
-        << "[wgpu section] added plane #" << section_planes_.size() - 1
-        << " origin=(" << point.x() << "," << point.y() << "," << point.z() << ")"
-        << " normal=(" << n.x() << "," << n.y() << "," << n.z() << ")";
-    if (isExposed()) requestUpdate();
-    return true;
+bool ViewportWindow::addSectionPlaneAtSurface(const Eigen::Vector3f& point, const Eigen::Vector3f& normal, float visual_radius) {
+    return core_.addSectionPlaneAtSurface(point, normal, visual_radius);
 }
 
-void ViewportWindow::removeSectionPlane(int index) {
-    if (index < 0 || index >= int(section_planes_.size())) return;
-    section_planes_.erase(section_planes_.begin() + index);
-    Log::info().noquote() << "[wgpu section] removed plane" << index;
-    if (isExposed()) requestUpdate();
-}
+void ViewportWindow::removeSectionPlane(int index) { core_.removeSectionPlane(index); }
 
-void ViewportWindow::clearSectionPlanes() {
-    if (section_planes_.empty()) return;
-    section_planes_.clear();
-    Log::info() << "[wgpu section] cleared all planes";
-    if (isExposed()) requestUpdate();
-}
+void ViewportWindow::clearSectionPlanes() { core_.clearSectionPlanes(); }
 
 void ViewportWindow::setOverlayLines(
         const std::vector<OverlayRenderer::LineGroup>& groups) {