Small tweak for using transparent+overlapping regions:

On mouse-over these regions now become active always (as if they were opaque).
This active state is used by many tools, or for drawing cursors.

Currently, all events (if not handled by button region) are passed on anyway to
the underlying region.

Visible errors were for example drawing the paint brush circle.

source/blender/editors/screen/screen_edit.c

@@ -1291,9 +1291,12 @@ void ED_screen_set_subwinactive(bContext *C, wmEvent *event)
 						break;
 		}
 		if (sa) {
+			/* make overlap active when mouse over */
 			for (ar = sa->regionbase.first; ar; ar = ar->next) {
-				if (BLI_rcti_isect_pt_v(&ar->winrct, &event->x))
+				if (BLI_rcti_isect_pt_v(&ar->winrct, &event->x)) {
 					scr->subwinactive = ar->swinid;
+					break;
+				}
 			}
 		}
 		else