tdf#92670 GtkSalGraphics::NWPaintGTKMenubar: fix modified clip rectangle

Happens only with when double buffering is enabled, and e.g. with the
oxygen GTK2 theme, but not with the default one.

Change-Id: Iad9911121e57388c84c25860413763f75e06b0ea

vcl/unx/gtk/gdi/salnativewidgets-gtk.cxx

@@ -3184,6 +3184,15 @@ bool GtkSalGraphics::NWPaintGTKToolbar(
     return true;
 }
 
+/// Converts a VCL Rectangle to a GdkRectangle.
+static void lcl_rectangleToGdkRectangle(const Rectangle& rRectangle, GdkRectangle& rGdkRectangle)
+{
+    rGdkRectangle.x = rRectangle.Left();
+    rGdkRectangle.y = rRectangle.Top();
+    rGdkRectangle.width = rRectangle.GetWidth();
+    rGdkRectangle.height = rRectangle.GetHeight();
+}
+
 bool GtkSalGraphics::NWPaintGTKMenubar(
             GdkDrawable* gdkDrawable,
             ControlType, ControlPart nPart,
@@ -3218,10 +3227,7 @@ bool GtkSalGraphics::NWPaintGTKMenubar(
 
     for( clipList::const_iterator it = rClipList.begin(); it != rClipList.end(); ++it )
     {
-        clipRect.x = it->Left();
-        clipRect.y = it->Top();
-        clipRect.width = it->GetWidth();
-        clipRect.height = it->GetHeight();
+        lcl_rectangleToGdkRectangle(*it, clipRect);
 
         // handle Menubar
         if( nPart == PART_ENTIRE_CONTROL )
@@ -3241,6 +3247,10 @@ bool GtkSalGraphics::NWPaintGTKMenubar(
                                 GTK_WIDGET(m_pWindow),
                                 "base",
                                 x, y, w, h );
+
+            // Do the conversion again, in case clipRect has been modified.
+            lcl_rectangleToGdkRectangle(*it, clipRect);
+
             gtk_paint_box( gWidgetData[m_nXScreen].gMenubarWidget->style,
                            gdkDrawable,
                            stateType,