After posting code using the Code button no line numbers appear and I can't double-click to highlight the posted code. See this thread. Everything works correctly in the original post to that thread, but not in mine where I used the Code button.

doesn't work as described in the Formatting Help button either, just putting either 4 spaces or one tab in front of each line.

Looks fine to me. Remember, code highlighting is Javascript based and it only initiates at the beginning of a page load, so if you post with quick reply, you will need to refresh before seeing it.