Drag & drop is started by dragging on the title bar of the tab thumbnail, just like on other window managers. Clicking on the thumbnail image content would activate the tab. Activating a tab is more frequent than drag&drop so the mouse operation is used by it. Dragging on the thumbnail image is the usual browser operation of dragging on an image of a webpage. The "disabled" icon is the browser's way of indicating image dragging is not applicable. Hope this help.