Fix pipe table parsing with a leading paragraph

[MihaZupan]

Closes #818
Addresses issues referenced in #885 (comment), #885 (comment)

This PR effectively reverts #885 as that change was swallowing whatever information was there in the paragraph before the table.
E.g. for

Some text
| A |
|---|
| B |

"Some text" would be lost.

Instead I relaxed the table validation to no longer enforce starting on the first line (localLineIndex > 0 and deltaLine > 0 checks), and also save the current block in the AST before we replace it with a table.

Given this is happening while we're parsing inlines, I don't think we can easily insert an extra block at the current layer, so I instead added the leading paragrah as the first child of the table, and the first element is special-cased in the renderer.
That is, instead of

document
  paragraph
    some text
  table
    A ...

we're instead producing

document
  table
    paragraph
      some text
    A ...

Since TableRow is a public type, this does risk breaking code that was walking the AST and assuming that all children of a Table are TableRows - this seems plausible given our own renderer did that.
I'm open to alternative suggestions.

[MihaZupan]

Alternative approach that'd get us the expected syntax tree shape and avoids modifying the renderer: master...MihaZupan:table-para2
This being a hack that says "insert the block, but skip processing it in MarkdownParser.ProcessInlines".

Could theoretically break someone that wanted to look at the table shape from a different parser (since the table isn't inserted until later), but that seems less likely?

[xoofx]

Could theoretically break someone that wanted to look at the table shape from a different parser (since the table isn't inserted until later), but that seems less likely?

Yes, maybe that workaround is ok. Table parsing is a bit awful unfortunately, so this new code doesn't change that 😅

[RickStrahl]

Can the same logic that's used for GridTable parsing to find the start of the block? GridTables seem to have the correct behavior.

ps. I haven't had time to look at the code beyond what was posted here, just going on the rendering behavior in the current 0.42 release.

[MihaZupan]

Maybe.
Grid tables are parsed with a block parser, while the pipe table is parsed with an inline one that effectively hijacks a paragraph block and replaces it with a table block.

I haven't been bored enough yet to look at what it'd take to turn it into a block parser instead 😄
Maybe Alexandre remembers more details.

[RickStrahl]

Yeah I know what you mean - I took a look at the code a few days ago thinking I could help in a hurry, but without setting aside a good chunk of time to understand the flow these block parsers are easy to mess up. I made a custom fix a few years ago and discovered months later it was breaking in completely unexpected ways.

Thanks for your effort here - really appreciate you taking the time!

Aloha.

[MihaZupan]

Note that this change makes it work when you have leading text before the table.
If you have text right after the table, that still won't work.