Opera has a annoying habit of popping up a new Transfers tab, everytime you start a file download, this could be intrusive at times, since this tab may be set to steal focus on the browser, bring it to the foreground.
If you are looking to disable the transfers tab or open it in the background, you can follow the steps listed in this post.
Step 1: Click on the Tools > Transfers or use the shortcut key Ctrl + J to open the Transfers tab.
Step 2: In the Transfers tab, click on the view button to open up the options and select, Never Show When Starting Download, if you want to completely disable the Transfers tab, or select Show in Background when starting downloads, if you want to see the Transfers window, but do not want it to steal focus and come into the foreground.
Disabling the Transfers tab is just for convenience, you can always view the transfers window by using the Ctrl + J shortcut key.