When setting the editing menu visible on the omnibox, the first time, it is shown then hidden again instantly afterwards. This behavior occurs again any time the first responder changes (for example if focusing a textbox in the webpage). The menu is (by apple's design) hidden when the first responder changes, so this is probably related.
By delaying showing the menu for a runloop after setting the first responder, behavior is correct. Investigate why this workaround is necessary.