Easy block selection with mouse
From Vim Tips Wiki
created 2006 · complexity basic · author Georg Dahn · version 6.0
In some editors it is possible to make a block selection by holding the Alt-key and the left mouse key. Since <M-LeftMouse> is not used in Vim, nor is <M-LeftDrag>, it is reasonable to enable this behavior in Vim, too. To do this, just add the following mappings to your vimrc file:
noremap <M-LeftMouse> <4-LeftMouse> inoremap <M-LeftMouse> <4-LeftMouse> onoremap <M-LeftMouse> <C-C><4-LeftMouse> noremap <M-LeftDrag> <LeftDrag> inoremap <M-LeftDrag> <LeftDrag> onoremap <M-LeftDrag> <C-C><LeftDrag>
At least on gnome the alt+mouse-left key is used to move the application window around.
On X Servers in general that is alt+left mouse, but no one prevents you from using any mapping unused for you to do the same if you need it