If you are using the web based client, then the best solution would be to contact the Yahoo help desk. It may be a quirk in their programming and nothing to do with your system. Then again, it may be some other browser extension getting in the way, there's not a lot to go on. As Specaircrew mentioned, changing the browser might help, disabling add-ons and extensions within the browser could be an option (temporarily, just to find out if one of them is the culprit), you could try a 'private' window to see if that makes a difference.