Repro steps:
1) Open up a web page (e.g. google.com)
2) Single-click the omnibox to focus it. The URL is highlighted but not expanded.
3) Double-click to highlight a word (e.g. double-click the 'c' in 'com')
4) The word where the mouse is after, rather than before, expanding is highlighted (e.g. the 'www')

If you skip step 2 then it works as intended. Note explicitly focusing the omnibox is not required, e.g. it seems it can gain focus after navigation.