popover_menus: Fix the API for popover centering.

The offset modifier is tricky and its calculations is related to the
placement of the popover. So, to prevent inconsistency, we should fix
the placement to some value.

web/src/popover_menus.js

@@ -204,6 +204,7 @@ function get_props_for_popover_centering(popover_props) {
             left: reference_offset,
             top: reference_offset,
         }),
+        placement: "top",
         popperOptions: {
             modifiers: [
                 {