When text is output by a program, that’s the output—fin.
To reflow a program’s output would require
Regenerating it; or
Piping it to a formatting program; or
Copying it into an editor that can reflow.
Once it’s on the screen, you won’t have much luck changing how it displays.
You can set mouse= to disable mouse entirely, use set mouse=a to restore mouse behaviour. You can automatically enable/disable mouse only in terminal vim but not in GUI vim by checking for if has("gui"), or by putting your config in .gvimrc.
Alternatively, you can get into the habit of changing window using Alt-Tab instead of the mouse. As a side effect, ...
You should simply remove that undo file, it's unlikely you'll manage to get something useful from it.
You can actually trim the undo history by setting 'undolevels' (see also :help clear-undo), but when you do so, the newer changes are preserved and the older ones are purged. If the newer changes are the problematic ones, trimming them means trimming the ...
:nnoremap <Down> <Nop>
:nnoremap <Up> <Nop>
does the trick (in conjunction with :set mouse=). Of course, you can't use the arrow keys any more, but if you're using vim, you didn't care anyway.
Edit: Need to this in insert mode too, so add the following:
:inoremap <Down> <Nop>
:inoremap <Up> <Nop>
You can use a couple of methods to achieve the same end result as your request.
You can just open a vertical split and then move it to leftmost position using Controlw + ShiftH (note that the "h" is uppercase.) See :help CTRL-W_H for more information.
You can use :topleft to modify your :vs command to open the window in the leftmost position. You can use ...