Explicitly set current border width when BS_PIXEL

When a window becomes managed, if the config specifies a pixel border
style, explicitly set the current border width so it does not change
when the container changes from tiling to floating.

fixes #1266

src/manage.c

@@ -450,6 +450,12 @@ void manage_window(xcb_window_t window, xcb_get_window_attributes_cookie_t cooki
         }
     }
 
+    if (nc->border_style == BS_PIXEL) {
+        /* if the border style is BS_PIXEL, explicitly set the border width of
+         * the new container */
+        nc->current_border_width = (want_floating ? config.default_floating_border_width : config.default_border_width);
+    }
+
     /* to avoid getting an UnmapNotify event due to reparenting, we temporarily
      * declare no interest in any state change event of this window */
     values[0] = XCB_NONE;