Windows: avoid changing the current Fl_Surface_Device when drawing an…

… Fl_Double_Window.

git-svn-id: file:///fltk/svn/fltk/branches/branch-1.4@12792 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

src/drivers/WinAPI/Fl_WinAPI_Window_Driver.cxx

@@ -307,12 +307,30 @@ void Fl_WinAPI_Window_Driver::flush_double()
   }
   if (pWindow->damage() & ~FL_DAMAGE_EXPOSE) {
     fl_clip_region(i->region); i->region = 0;
+#if 0 /* Short form that transiently changes the current Fl_Surface_Device */
     fl_begin_offscreen(other_xid);
     fl_graphics_driver->clip_region( 0 );
     draw();
     fl_end_offscreen();
+#else
+    /* Alternative form that avoids changing the current Fl_Surface_Device.
+     The code run in the window draw() method can call Fl_Surface_Device::surface()
+     and conclude that it's drawing to the display, which is ultimately true
+     for an Fl_Double_Window.
+     */
+    HDC sgc = fl_gc;
+    Window save_w = fl_window; fl_window = (Window)other_xid;
+    fl_gc = fl_makeDC(other_xid);
+    int savedc = SaveDC(fl_gc);
+    fl_graphics_driver->gc(fl_gc);
+    fl_graphics_driver->restore_clip(); // duplicate clip region into new gc
+    draw();
+    RestoreDC(fl_gc, savedc);
+    DeleteDC(fl_gc);
+    fl_window = save_w;
+    fl_graphics_driver->gc(sgc);
+#endif
   }
-
   int X,Y,W,H; fl_clip_box(0,0,w(),h(),X,Y,W,H);
   if (other_xid) fl_copy_offscreen(X, Y, W, H, other_xid, X, Y);
 }