This is a years-old problem in Opera. There's a discussion thread about a fix for Windows here, but the solution does not work on OS X - the file gets overwritten on re-start. Hopefully, the values written into the file are stored somewhere else that can also be edited.
I know that can be a problem when the entire width of the window is filled up with tabs and there is no place to grab the window for moving it, but this isn't rocket science. I am sure the creative folks at Opera can come up with a solution, maybe a little handlebar that pops up when you move your cursor to the top right edge of the browser.