The Design tab's MCP socket must not wait for someone to click the tab

Building the Design tab on first use (1750c52211) leaves m_design_panel null
until a human selects the tab. McpControl::handle_on_main refused every verb
while it was null, so start_mcp_control_if_enabled() opened the socket and
then answered "Design panel not ready" to everything — for the whole session
if nobody clicked. That is precisely the headless case the socket exists for:
the click-test rig drives this app over it with no window manager and no user.

MainFrame::ensure_design_panel() now builds the panel on demand and returns
it; the tab activation and the MCP dispatcher both go through it, so there is
one construction site rather than two. Safe to build wx controls there: that
handler is dispatched on the main thread by CallAfter, as its own comment
says.

The startup saving is untouched — a launch that never opens the tab and never
speaks MCP still builds nothing.

Verified: libslic3r_gui builds clean 745/745; fork parity green with snaporca,
which carries the same fix (McpControl.cpp is a byte-identical shared source).

src/slic3r/GUI/CAD/McpControl.cpp

@@ -1990,9 +1990,12 @@ json action_set_feature_expr(DesignPanel* panel, const json& params)
 std::string handle_on_main(const std::string& method, const json& params, const json& id)
 {
     MainFrame* mf = wxGetApp().mainframe;
-    if (!mf || !mf->m_design_panel)
+    // The panel is built on first use, and in a headless session nobody clicks the tab that
+    // would build it -- so build it here rather than refusing. Safe: this runs on the main
+    // thread (see the CallAfter that dispatches us).
+    DesignPanel* panel = mf ? mf->ensure_design_panel() : nullptr;
+    if (!panel)
         return rpc_error(id, -32001, "Design panel not ready");
-    DesignPanel* panel = mf->m_design_panel;
 
     // Stale-id guard, checked here rather than in each handler.
     //

src/slic3r/GUI/MainFrame.cpp

@@ -1245,6 +1245,19 @@ void MainFrame::show_option(bool show)
     }
 }
 
+#ifdef SLIC3R_CAD
+DesignPanel* MainFrame::ensure_design_panel()
+{
+    if (m_design_panel == nullptr && m_design_page != nullptr) {
+        wxBusyCursor busy;
+        m_design_panel = new DesignPanel(m_design_page);
+        m_design_page->GetSizer()->Add(m_design_panel, 1, wxEXPAND);
+        m_design_page->Layout();
+    }
+    return m_design_panel;
+}
+#endif
+
 void MainFrame::init_tabpanel() {
     // wxNB_NOPAGETHEME: Disable Windows Vista theme for the Notebook background. The theme performance is terrible on
     // Windows 10 with multiple high resolution displays connected.
@@ -1290,12 +1303,7 @@ void MainFrame::init_tabpanel() {
             // Built on first activation, never at startup: the panel creates several hundred
             // controls and its own GL canvas, which a user who does not open the tab should
             // not pay for.
-            if (m_design_panel == nullptr) {
+            ensure_design_panel();
-                wxBusyCursor busy;
-                m_design_panel = new DesignPanel(m_design_page);
-                m_design_page->GetSizer()->Add(m_design_panel, 1, wxEXPAND);
-                m_design_page->Layout();
-            }
             // Re-sync the Design bed to the active printer: the panel is built before the
             // printer profile is fully applied, so its bed must refresh on activation or the
             // grid (true bed) spills past the stale default bed quad.

src/slic3r/GUI/MainFrame.hpp

@@ -391,6 +391,11 @@ public:
     // selected, so everything the Design panel builds stays off the startup path.
     wxPanel*              m_design_page { nullptr };
     DesignPanel*          m_design_panel { nullptr };
+    // Builds the Design panel if it does not exist yet and returns it (null only before the
+    // placeholder page itself exists). Main thread only -- it creates wx controls. Both the
+    // tab activation and the MCP socket go through this: the socket is driven headlessly,
+    // with nobody to click the tab, and without this every verb would answer "not ready".
+    DesignPanel*          ensure_design_panel();
 #endif
     //BBS: GUI refactor
     MonitorPanel*         m_monitor{ nullptr };