feat(wasm-dom): tests for context menus + scrollbars; fix clean wx build

- Standalone wx test apps (contextmenu, scrollbar) + e2e specs proving the
  wxScrollBar control, the wxScrolledWindow gutter, and DoPopupMenu.
- KiCad in-app specs: pl_editor + pcbnew canvas right-click context menus and
  scrolled-panel scrollbar gutters, with baseline screenshots.
- build-wx-wasm.sh: serial -j1 fallback when the parallel build trips the
  intermittent clean-build race (a generated header read mid-rewrite by
  concurrent compiles; never happens at -j1). Recovery resumes from built
  objects; a genuine error still fails the serial pass.
- Bump wxwidgets submodule (context menus + draggable scrollbars).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/apps/standalone/scrollbar/scrollbar_test.cpp

@@ -0,0 +1,122 @@
+// Scrollbar Test - draggable scrollbars in the WASM DOM port. Exercises BOTH
+// the standalone wxScrollBar control AND a wxScrolledWindow's built-in gutters
+// (the kind KiCad's scrolled panels use).
+
+#include "wx/wxprec.h"
+
+#ifndef WX_PRECOMP
+    #include "wx/wx.h"
+#endif
+
+#include "wx/scrolbar.h"
+#include "wx/scrolwin.h"
+
+#ifdef __EMSCRIPTEN__
+#include <emscripten/emscripten.h>
+#endif
+
+class ScrollApp : public wxApp
+{
+public:
+    virtual bool OnInit() override;
+};
+
+class ScrollFrame : public wxFrame
+{
+public:
+    ScrollFrame();
+
+private:
+    wxScrollBar* m_vbar = nullptr;
+    wxScrollBar* m_hbar = nullptr;
+    wxScrolledWindow* m_scrolled = nullptr;
+    wxStaticText* m_status = nullptr;
+
+    void Log(const wxString& msg);
+    void OnBarScroll(wxScrollEvent& evt);
+};
+
+wxIMPLEMENT_APP(ScrollApp);
+
+bool ScrollApp::OnInit()
+{
+    if (!wxApp::OnInit())
+        return false;
+
+    (new ScrollFrame())->Show(true);
+    return true;
+}
+
+ScrollFrame::ScrollFrame()
+    : wxFrame(nullptr, wxID_ANY, "Scrollbar WASM Test",
+              wxDefaultPosition, wxSize(700, 560))
+{
+    wxBoxSizer* root = new wxBoxSizer(wxVERTICAL);
+
+    m_status = new wxStaticText(this, wxID_ANY, "Drag a scrollbar");
+    root->Add(m_status, 0, wxALL, 8);
+
+    // --- Standalone wxScrollBar controls (one of each orientation) ---
+    wxBoxSizer* bars = new wxBoxSizer(wxHORIZONTAL);
+
+    m_vbar = new wxScrollBar(this, wxID_ANY, wxDefaultPosition,
+                             wxSize(20, 140), wxSB_VERTICAL);
+    m_vbar->SetScrollbar(0, 20, 100, 20);
+    bars->Add(m_vbar, 0, wxALL, 8);
+
+    m_hbar = new wxScrollBar(this, wxID_ANY, wxDefaultPosition,
+                             wxSize(240, 20), wxSB_HORIZONTAL);
+    m_hbar->SetScrollbar(0, 20, 100, 20);
+    bars->Add(m_hbar, 0, wxALL | wxALIGN_CENTRE_VERTICAL, 8);
+
+    root->Add(bars, 0, wxEXPAND);
+
+    m_vbar->Bind(wxEVT_SCROLL_THUMBTRACK, &ScrollFrame::OnBarScroll, this);
+    m_vbar->Bind(wxEVT_SCROLL_CHANGED, &ScrollFrame::OnBarScroll, this);
+    m_hbar->Bind(wxEVT_SCROLL_THUMBTRACK, &ScrollFrame::OnBarScroll, this);
+    m_hbar->Bind(wxEVT_SCROLL_CHANGED, &ScrollFrame::OnBarScroll, this);
+
+    // --- wxScrolledWindow with overflowing content (built-in gutters) ---
+    m_scrolled = new wxScrolledWindow(this, wxID_ANY, wxDefaultPosition,
+                                      wxDefaultSize,
+                                      wxVSCROLL | wxHSCROLL | wxBORDER_SUNKEN);
+    m_scrolled->SetScrollRate(10, 10);
+
+    wxPanel* content = new wxPanel(m_scrolled, wxID_ANY);
+    content->SetBackgroundColour(wxColour(230, 235, 250));
+    content->SetMinSize(wxSize(1400, 1400)); // far larger than the viewport
+    wxBoxSizer* cs = new wxBoxSizer(wxVERTICAL);
+    cs->Add(new wxStaticText(content, wxID_ANY,
+            "Scrolled content (drag the gutter on the right/bottom edge)"),
+            0, wxALL, 12);
+    content->SetSizer(cs);
+
+    wxBoxSizer* inner = new wxBoxSizer(wxVERTICAL);
+    inner->Add(content, 1, wxEXPAND);
+    m_scrolled->SetSizer(inner);
+    m_scrolled->FitInside();
+
+    root->Add(m_scrolled, 1, wxEXPAND | wxALL, 8);
+
+    SetSizer(root);
+
+    Log("app started");
+}
+
+void ScrollFrame::Log(const wxString& msg)
+{
+    if (m_status)
+        m_status->SetLabel(msg);
+
+#ifdef __EMSCRIPTEN__
+    EM_ASM({
+        console.log('[SCROLLBAR_EVENT] ' + UTF8ToString($0));
+    }, (const char*)msg.utf8_str());
+#endif
+}
+
+void ScrollFrame::OnBarScroll(wxScrollEvent& evt)
+{
+    Log(wxString::Format("scrollbar pos %d", evt.GetPosition()));
+    evt.Skip();
+}