TextToFlowed doesn't generate valid flowed text

[jessicah]

Describe the bug
Using TextToFlowed to generate format=flowed output, it doesn't appear to generate valid output. Lines appear to be space-stuffed at the beginning.

Platform (please complete the following information):

• OS: Windows
• .NET Runtime: Core
• .NET Framework: .NET Core 3.1
• MimeKit Version: 2.6.0

To Reproduce
Steps to reproduce the behavior:

var flowedConverter = new MimeKit.Text.TextToFlowed();
var flowedText = flowedConverter.Convert(textWithParagraphsInSingleLines);

Expected behavior
Text should be correctly flowed with long lines broken up with proper line continuations.

[jessicah]

I think it might be this line: https://github.com/jstedfast/MimeKit/blob/master/MimeKit/Text/TextToFlowed.cs#L133

As it doesn't increment the index, the trailing space after splitting the line is processed again on subsequent calls to GetFlowedLine: https://github.com/jstedfast/MimeKit/blob/master/MimeKit/Text/TextToFlowed.cs#L130.

I believe the correct thing to do here is to increment index until a non-space character is encountered.

[jstedfast]

Do you happen to have a test case that I can use to see the issue and verify that it's fixed once it's fixed?

[jstedfast]

FWIW, so that we are all on the same page with regards to expected behavior, the specification for format=flowed is: https://tools.ietf.org/html/rfc3676

I just added a test case (it's split between 2 commits because I can't get VS2019 to run the unit tests for some reason so had to commit a partial test in order to complete it on my MacBook w/ VS4Mac which runs them fine).

b4a92eb adds the expected output which I added a comment to which exposes at least 1 bug but I don't think that's the bug that you are hitting.

Secondly, keep in mind that according to my docs for TextToFlowed, the output is designed for format=flowed; delsp=yes which has a different expected output than delsp=no.

Maybe TextToFlowed should be more configurable to dictate what delsp behavior to use.

I saw your fork & fix and I'm not sure that your fix is correct because because if you set delsp=yes, then your fix looks like it would break FlowedToText when DeleteSpace=true.

[jstedfast]

I'm not sure if my fix solves your issue, but if I understand the issue you described correctly, then it does. It also fixes other bugs that I found.