Small drawing glitch, for UI with transparent overlapping regions;

- The corner 'duplicate/merge area' widget was drawing too often.

(Added comment in subwindow 'active' code, too many redraws are being
sent on moving mouse over region eddges)

source/blender/editors/screen/screen_edit.c

@@ -1302,6 +1302,7 @@ void ED_screen_set_subwinactive(bContext *C, wmEvent *event)
 			screen_cursor_set(win, event);
 		}
 		else {
+			/* notifier invokes freeing the buttons... causing a bit too much redraws */
 			if (oldswin != scr->subwinactive) {
 				region_cursor_set(win, scr->subwinactive, TRUE);
 				WM_event_add_notifier(C, NC_SCREEN | ND_SUBWINACTIVE, scr);

source/blender/windowmanager/intern/wm_draw.c

@@ -606,6 +606,7 @@ static void wm_method_draw_triple(bContext *C, wmWindow *win)
 
 		for (ar = sa->regionbase.first; ar; ar = ar->next) {
 			if (ar->swinid && ar->do_draw) {
+				
 				if (ar->overlap == 0) {
 					CTX_wm_region_set(C, ar);
 					ED_region_do_draw(C, ar);
@@ -635,7 +636,6 @@ static void wm_method_draw_triple(bContext *C, wmWindow *win)
 			if (ar->swinid && ar->overlap) {
 				CTX_wm_region_set(C, ar);
 				ED_region_do_draw(C, ar);
-				ED_area_overdraw_flush(sa, ar);
 				CTX_wm_region_set(C, NULL);
 				
 				wm_draw_region_blend(win, ar);