add back autocmd for pandoc setup after colorscheme switch

as kindly pointed out by @chrisbra

syntax/pandoc.vim

@@ -705,6 +705,33 @@ function! s:SetupPandocHighlights()
   hi def link pandocHRule Delimiter
 endfunction
 
+" Whenever the colorscheme changes, all highlights are cleared.
+"
+" The most common circumstance is that the vimrc picks a colorscheme *at
+" startup*, then a file is opened and the syntax is set based on that file. So
+" the most common situation is that the colorscheme runs, then the syntax
+" runs, and that's that. So if the code for the syntax (e.g., this code here
+" in vim-pandoc-syntax) *adds* new highlighting groups that weren't defined in
+" the colorscheme, that's almost always fine because the colorscheme rarely
+" changes after startup.
+"
+" But the colorscheme *can* change after startup. This happens for example any
+" time the user toggles their background (:set bg=light or :set bg=dark), or
+" picks another colorscheme (:colorscheme something_else). In these cases, the
+" new colorscheme calls `:highlight clear`, clearing any custom pandoc
+" highlighting groups.
+"
+" The solution is to register an autocommand that runs whenever the
+" ColorScheme changes, so that we can re-register vim-pandoc-syntax's custom
+" highlighting groups, after the new colorscheme has cleared them.
+"
+" (This also affects popular plugins like goyo.vim, which call `:colorscheme`
+" with your chosen colorscheme to approximate undoing any custom highlighting
+" modifications that they've made.)
+augroup vim-pandoc-syntax
+  autocmd!
+  autocmd ColorScheme * call s:SetupPandocHighlights()
+augroup end
 call s:SetupPandocHighlights()
 
 " }}}1