OnActivate handler to:

1) Process delayed error messages from background processing
2) Set keyboard focus to the 3D scene if no wx window has keyboard focus.

src/slic3r/GUI/ConfigWizard.cpp

@@ -65,9 +65,9 @@ PrinterPicker::PrinterPicker(wxWindow *parent, const VendorProfile &vendor, wxSt
 
     auto *sizer = new wxBoxSizer(wxVERTICAL);
 
-    const auto font_title = GetFont().MakeBold().Scaled(1.3);
+    const auto font_title = GetFont().MakeBold().Scaled(1.3f);
     const auto font_name = GetFont().MakeBold();
-    const auto font_alt_nozzle = GetFont().Scaled(0.9);
+    const auto font_alt_nozzle = GetFont().Scaled(0.9f);
 
     // wxGrid appends widgets by rows, but we need to construct them in columns.
     // These vectors are used to hold the elements so that they can be appended in the right order.
@@ -789,7 +789,7 @@ void ConfigWizardIndex::on_mouse_move(wxMouseEvent &evt)
 
     const ssize_t item_hover_new = pos.y / item_height();
 
-    if (item_hover_new < items.size() && item_hover_new != item_hover) {
+	if (item_hover_new < ssize_t(items.size()) && item_hover_new != item_hover) {
         item_hover = item_hover_new;
         Refresh();
     }

src/slic3r/GUI/MainFrame.cpp

@@ -105,6 +105,11 @@ wxFrame(NULL, wxID_ANY, SLIC3R_BUILD, wxDefaultPosition, wxDefaultSize, wxDEFAUL
         event.Skip();
     });
 
+    Bind(wxEVT_ACTIVATE, [this](wxActivateEvent& event) {
+        if (m_plater != nullptr && event.GetActive())
+            m_plater->on_activate();
+    });
+
     wxGetApp().persist_window_geometry(this);
 
     update_ui_from_settings();    // FIXME (?)

src/slic3r/GUI/Plater.cpp

@@ -962,6 +962,7 @@ struct Plater::priv
     std::atomic<bool>           arranging;
     std::atomic<bool>           rotoptimizing;
     bool                        delayed_scene_refresh;
+    std::string                 delayed_error_message;
 
     wxTimer                     background_process_timer;
 
@@ -1947,6 +1948,8 @@ unsigned int Plater::priv::update_background_process(bool force_validation)
     this->background_process_timer.Stop();
     // Update the "out of print bed" state of ModelInstances.
     this->update_print_volume_state();
+    // The delayed error message is no more valid.
+    this->delayed_error_message.clear();
     // Apply new config to the possibly running background task.
     bool               was_running = this->background_process.running();
     Print::ApplyStatus invalidated = this->background_process.apply(this->q->model(), wxGetApp().preset_bundle->full_config());
@@ -1985,8 +1988,18 @@ unsigned int Plater::priv::update_background_process(bool force_validation)
                 return_state |= UPDATE_BACKGROUND_PROCESS_RESTART;
         } else {
             // The print is not valid.
-            // The error returned from the Print needs to be translated into the local language.
-            GUI::show_error(this->q, _(err));
+            // Only show the error message immediately, if the top level parent of this window is active.
+            auto p = dynamic_cast<wxWindow*>(this->q);
+            while (p->GetParent())
+                p = p->GetParent();
+            auto *top_level_wnd = dynamic_cast<wxTopLevelWindow*>(p);
+            if (top_level_wnd && top_level_wnd->IsActive()) {
+                // The error returned from the Print needs to be translated into the local language.
+                GUI::show_error(this->q, _(err));
+            } else {
+                // Show the error message once the main window gets activated.
+                this->delayed_error_message = _(err);
+            }
             return_state |= UPDATE_BACKGROUND_PROCESS_INVALID;
         }
     }
@@ -3095,6 +3108,24 @@ void Plater::on_config_change(const DynamicPrintConfig &config)
         this->p->schedule_background_process();
 }
 
+void Plater::on_activate()
+{
+    wxWindow *focus_window = wxWindow::FindFocus();
+    if (focus_window == nullptr) {
+		// Activating the main frame, and no window has keyboard focus.
+		// Set the keyboard focus to the visible Canvas3D.
+        if (this->p->view3D->IsShown())
+			this->p->view3D->get_wxglcanvas()->SetFocus();
+		else if (this->p->preview->IsShown())
+			this->p->preview->get_wxglcanvas()->SetFocus();
+    }
+    if (! this->p->delayed_error_message.empty()) {
+		std::string msg = std::move(this->p->delayed_error_message);
+		this->p->delayed_error_message.clear();
+        GUI::show_error(this, msg);
+	}
+}
+
 const wxString& Plater::get_project_filename() const
 {
     return p->project_filename;

src/slic3r/GUI/Plater.hpp

@@ -151,6 +151,8 @@ public:
 
     void on_extruders_change(int extruders_count);
     void on_config_change(const DynamicPrintConfig &config);
+    // On activating the parent window.
+    void on_activate();
 
     void update_object_menu();