A maximized window cannot be dragged in most window managers.

The area above the tabs where you can click to drag or double-click to restore is pretty narrow - people who want to run Opera maximized prefer it that way so the tabs go all the way to the top of the screen. Since you don't appear to want to run Opera maximized, you could go to opera:config and set "Chrome Integration Drag Area Maximized" to a larger number (at least 5) to make it easier to find the right area.