Ensure that wxPreviewCanvas always gets idle event.

Use wxWS_EX_PROCESS_IDLE flag to ensure that the print preview receives idle
events even when wxIDLE_PROCESS_SPECIFIED global idle mode is used.

Closes #15104.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76652 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/prntbase.cpp

@@ -855,6 +855,10 @@ wxPreviewCanvas::wxPreviewCanvas(wxPrintPreviewBase *preview, wxWindow *parent,
                                  const wxPoint& pos, const wxSize& size, long style, const wxString& name):
 wxScrolledWindow(parent, wxID_ANY, pos, size, style | wxFULL_REPAINT_ON_RESIZE, name)
 {
+    // As we rely on getting idle events, do this to ensure that we do receive
+    // them even when using wxIDLE_PROCESS_SPECIFIED global idle mode.
+    SetExtraStyle(wxWS_EX_PROCESS_IDLE);
+
     m_printPreview = preview;
 #ifdef __WXMAC__
     // The app workspace colour is always white, but we should have