man.vim: Fix very long justified lines #9023

When nroff justifies a line, it fills the line with whitespace to meet
$MANWIDTH.  With $MANWIDTH=9999, that of course results in nonsense (and
behaves poorly with 'cursorline' option).

To work around that, instead of trying to hard-justify the lines, just
replace the mega-whitespace with a fixed size of 10 spaces.

Perhaps N/Vim needs a "soft justify" feature?

runtime/autoload/man.vim

@@ -167,6 +167,10 @@ function! s:put_page(page) abort
   while getline(1) =~# '^\s*$'
     silent keepjumps 1delete _
   endwhile
+  " XXX: nroff justifies text by filling it with whitespace.  That interacts
+  " badly with our use of $MANWIDTH=9999.  Hack around this by using a fixed
+  " size for those whitespace regions.
+  silent! keeppatterns keepjumps %s/\s\{999,}/\=repeat(' ', 10)/g
   lua require("man").highlight_man_page()
   setlocal filetype=man
 endfunction