Don't allow DestroyClippingRegion() to remove paint update area clipping with GTK3

See #18560

include/wx/gtk/dc.h

@@ -63,8 +63,13 @@ public:
 
 class wxPaintDCImpl: public wxGTKCairoDCImpl
 {
+    typedef wxGTKCairoDCImpl BaseType;
 public:
     wxPaintDCImpl(wxPaintDC* owner, wxWindow* window);
+    virtual void DestroyClippingRegion() wxOVERRIDE;
+
+private:
+    const wxRegion& m_clip;
 
     wxDECLARE_NO_COPY_CLASS(wxPaintDCImpl);
 };

src/gtk/dc.cpp

@@ -337,6 +337,7 @@ wxClientDCImpl::wxClientDCImpl(wxClientDC* owner, wxWindow* window)
 
 wxPaintDCImpl::wxPaintDCImpl(wxPaintDC* owner, wxWindow* window)
     : wxGTKCairoDCImpl(owner, window)
+    , m_clip(window->m_nativeUpdateRegion)
 {
     cairo_t* cr = window->GTKPaintContext();
     wxCHECK_RET(cr, "using wxPaintDC without being in a native paint event");
@@ -345,6 +346,18 @@ wxPaintDCImpl::wxPaintDCImpl(wxPaintDC* owner, wxWindow* window)
     gc->EnableOffset(m_contentScaleFactor <= 1);
     SetGraphicsContext(gc);
 }
+
+void wxPaintDCImpl::DestroyClippingRegion()
+{
+    BaseType::DestroyClippingRegion();
+
+    // re-establish clip for paint update area
+    int x, y, w, h;
+    m_clip.GetBox(x, y, w, h);
+    cairo_t* cr = static_cast<cairo_t*>(GetCairoContext());
+    cairo_rectangle(cr, x, y, w, h);
+    cairo_clip(cr);
+}
 //-----------------------------------------------------------------------------
 
 wxScreenDCImpl::wxScreenDCImpl(wxScreenDC* owner)

src/gtk/window.cpp

@@ -5074,6 +5074,7 @@ void wxWindowGTK::GTKSendPaintEvents(const GdkRegion* region)
 
     m_paintContext = cr;
     m_updateRegion = wxRegion(int(x1), int(y1), int(x2 - x1), int(y2 - y1));
+    m_nativeUpdateRegion = m_updateRegion;
 #else // !__WXGTK3__
     m_updateRegion = wxRegion(region);
 #if wxGTK_HAS_COMPOSITING_SUPPORT