If the documents are not visible side by side, first move the mouse pointer to the button of the target document while keeping the mouse button pressed. The document in question is then displayed and you can move the mouse pointer into the document.
マウスボタンを押したままの状態で、マウスポインタが図を吸い込むようにクリップボードに読み込むまでしばらく待ちます。