diff --git a/src/slic3r/GUI/GUI_ObjectList.cpp b/src/slic3r/GUI/GUI_ObjectList.cpp
index 07119b8de..decd55dd2 100644
--- a/src/slic3r/GUI/GUI_ObjectList.cpp
+++ b/src/slic3r/GUI/GUI_ObjectList.cpp
@@ -3961,7 +3961,23 @@ void ObjectList::fix_through_netfabb()
 
 void ObjectList::simplify()
 {
-    GLGizmosManager& gizmos_mgr = wxGetApp().plater()->canvas3D()->get_gizmos_manager();
+    auto plater = wxGetApp().plater();
+    GLGizmosManager& gizmos_mgr = plater->canvas3D()->get_gizmos_manager();
+
+    // Do not simplify when a gizmo is open. There might be issues with updates
+    // and what is worse, the snapshot time would refer to the internal stack.
+    auto current_type = gizmos_mgr.get_current_type();
+    if (current_type == GLGizmosManager::Simplify) {
+        // close first
+        gizmos_mgr.open_gizmo(GLGizmosManager::EType::Simplify);
+    }else if (current_type != GLGizmosManager::Undefined) {
+        plater->get_notification_manager()->push_notification(
+            NotificationType::CustomSupportsAndSeamRemovedAfterRepair,
+            NotificationManager::NotificationLevel::RegularNotification,
+            _u8L("ERROR: Please close all manipulators available from "
+                 "the left toolbar before start simplify the mesh."));
+        return;
+    }
     gizmos_mgr.open_gizmo(GLGizmosManager::EType::Simplify);
 }
 
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoSimplify.cpp b/src/slic3r/GUI/Gizmos/GLGizmoSimplify.cpp
index 06a999cad..a921a121e 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoSimplify.cpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoSimplify.cpp
@@ -1,17 +1,14 @@
-// Include GLGizmoBase.hpp before I18N.hpp as it includes some libigl code, which overrides our localization "L" macro.
 #include "GLGizmoSimplify.hpp"
 #include "slic3r/GUI/GLCanvas3D.hpp"
 #include "slic3r/GUI/GUI_App.hpp"
-#include "slic3r/GUI/Plater.hpp"
 #include "slic3r/GUI/GUI_ObjectManipulation.hpp"
 #include "slic3r/GUI/GUI_ObjectList.hpp"
+#include "slic3r/GUI/NotificationManager.hpp"
+#include "slic3r/GUI/Plater.hpp"
 #include "libslic3r/AppConfig.hpp"
 #include "libslic3r/Model.hpp"
 #include "libslic3r/QuadricEdgeCollapse.hpp"
 
-#include <chrono>
-#include <thread>
-
 namespace Slic3r::GUI {
 
 GLGizmoSimplify::GLGizmoSimplify(GLCanvas3D &       parent,
@@ -38,7 +35,6 @@ bool GLGizmoSimplify::on_init()
     return true;
 }
 
-
 std::string GLGizmoSimplify::on_get_name() const
 {
     return (_L("Simplify")).ToUTF8().data();
@@ -195,6 +191,11 @@ void GLGizmoSimplify::on_render_input_window(float x, float y, float bottom_limi
                 process();
             } else {
                 // use preview and close
+                if (m_original_its.has_value()) {
+                    // fix hollowing, sla support points, modifiers, ...
+                    auto plater = wxGetApp().plater();
+                    plater->changed_mesh(m_obj_index);
+                }
                 close();
             }
         }
@@ -213,18 +214,17 @@ void GLGizmoSimplify::on_render_input_window(float x, float y, float bottom_limi
 
     if (m_need_reload) { 
         m_need_reload = false;
-
+        bool close_on_end = (m_state == State::close_on_end);
         // Reload visualization of mesh - change VBO, FBO on GPU
         m_parent.reload_scene(true);
-        if (m_state == State::close_on_end) {
+        // set m_state must be before close() !!!
+        m_state = State::settings;
+        if (close_on_end) {
             // fix hollowing, sla support points, modifiers, ...
             auto plater = wxGetApp().plater();
             plater->changed_mesh(m_obj_index);
             close(); 
         }
-
-        // change from simplifying | apply
-        m_state = State::settings;           
         
         // Fix warning icon in object list
         wxGetApp().obj_list()->update_item_error_icon(m_obj_index, -1);
@@ -287,7 +287,6 @@ void GLGizmoSimplify::process()
             m_last_error = max_error;
         } catch (SimplifyCanceledException &) {
             // set state out of main thread
-            m_last_error = {};
             m_state = State::settings; 
         }
         // need to render last status fn to change bar graph to buttons
@@ -310,10 +309,30 @@ bool GLGizmoSimplify::on_is_activable() const
 }
 
 void GLGizmoSimplify::on_set_state() 
-{ 
+{
     // Closing gizmo. e.g. selecting another one
     if (GLGizmoBase::m_state == GLGizmoBase::Off) {
-        m_volume = nullptr;            
+
+        // refuse outgoing during simlification
+        if (m_state != State::settings) {
+            GLGizmoBase::m_state = GLGizmoBase::On;
+            auto notification_manager = wxGetApp().plater()->get_notification_manager();
+            notification_manager->push_notification(
+                NotificationType::CustomNotification,
+                NotificationManager::NotificationLevel::RegularNotification,
+                _u8L("ERROR: Wait until Simplification ends or Cancel process."));
+            return;
+        }
+
+        // revert preview
+        if (m_original_its.has_value()) {
+            set_its(*m_original_its);
+            m_parent.reload_scene(true);
+            m_need_reload = false;
+        }
+
+        // invalidate selected model
+        m_volume = nullptr;
     }
 }
 
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoSimplify.hpp b/src/slic3r/GUI/Gizmos/GLGizmoSimplify.hpp
index 5a84cabad..4539fef01 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoSimplify.hpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoSimplify.hpp
@@ -1,14 +1,20 @@
 #ifndef slic3r_GLGizmoSimplify_hpp_
 #define slic3r_GLGizmoSimplify_hpp_
 
+// Include GLGizmoBase.hpp before I18N.hpp as it includes some libigl code,
+// which overrides our localization "L" macro.
 #include "GLGizmoBase.hpp"
-#include "libslic3r/Model.hpp"
+#include "admesh/stl.h" // indexed_triangle_set
 #include <thread>
 #include <optional>
 
 namespace Slic3r {
+
+class ModelVolume;
+
 namespace GUI {
 
+
 class GLGizmoSimplify : public GLGizmoBase
 {    
 public:
diff --git a/src/slic3r/GUI/Plater.cpp b/src/slic3r/GUI/Plater.cpp
index 22a1b36ae..e037a0ac8 100644
--- a/src/slic3r/GUI/Plater.cpp
+++ b/src/slic3r/GUI/Plater.cpp
@@ -4283,6 +4283,12 @@ bool Plater::priv::can_fix_through_netfabb() const
 
 bool Plater::priv::can_simplify() const
 {
+    // is object for simplification selected
+    if (get_selected_object_idx() < 0) return false;
+    // is already opened?
+    if (q->canvas3D()->get_gizmos_manager().get_current_type() ==
+        GLGizmosManager::EType::Simplify)
+        return false;
     return true;
 }