SCENARIO:

1. Use Opera browser. Issue was found on Opera 41.0.2353.46
2. Use Vector skin
3. On any wiki, open the edit form of a page (?action=edit)
4. On the edit form toolbar, expand 'Advanced' menu item
5. Click the 'Find&Replace' window on the right.

OBSERVED:
Nothing happens. I mean, really nothing. There are no messages logged in the browser's console log, there are no requests made. The click just does nothing.
The Find&Replace popup opens well and replaces text with no problem on other major browsers (MFfx, Chrome, IE10).