annoying missing feature. The amazing thing is that right click on the Serial input field allows to Select All of the serial input !! The option was forgtotten on the output.
My (cumbersome) workaround : select with the mouse the visible part; cut/paste to a text editor; scroll down and copy/paste again until the end of the output.
It is possible to display the Serial Monitor on the right side. That allows to see more lines at once.