Re: automatic redraw for multi-line syntax highlighting patterns

> the docs state (:help syn-multi-line) that syntax highlighting with
> multi-line patterns "mostly works as expected". I am not sure if this
> "mostly" includes automatic highlighting change of a multi-line syntax
> group when it's modified. Especially as soon as the pattern's end is
> destroyed. For example, take the commands of a session file ~/tmp/s.vim:
>
> call append(0, ['( comment?', ')'])syntax sync fromstarthi link m
> Commentsyntax match m /(.*\n)/
>
> They give us two lines highlighted as a comment (:hi Comment):
>
> ( comment?)
>
> Now, let's change the ')' into an 'x' via "2gg0rx". The test in the two
> lines does not match the defined syntax group anymore. But the display does
> not reflect this change right away. Instead I get something that's
> displayed partly as a comment (line 1) and partly as normal text (line 2):
>
> ( comment?)
>
> Note the different color of the two parentheses. I have to press CTRL-L to
> see the comment highlighting disappear completely. Having set syntax
> syncing to start from the very start of the file I would expect this to
> happen automatically. Is this imperfection the reason why the help says
> "mostly works" instead of "always works"? Or am I missing something?
>
> This is Vim 8.1.2337 (Huge version with GTK2 GUI) on Debian GNU/Linux,
> started with:
> vim --clean -S ~/tmp/s.vim
>
> Thanks in advance for all your insights,

When the text changes Vim by default assumes only that line needs to be
updated for syntax highlighting. In other words: the cached syntax
state at the start of the line is assumed to be valid.

If there are multi-line patterns, the sync mechanism needs to take care
of any syntax items where more state becomes invalid. The extreme
version is to use "syn sync fromstart". See ":help syn-sync".

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