CJK punctuation at the start of paragraphs are not adjusted sometimes

[balthild]

Description

I'm not sure if this is a bug or a feature that haven't been implemented. In #954, I see that CJK right puncts appearing at line start are compressed:

However, I found that there're some cases these puncts are not compressed when they appear at paragraph start:

This causes the start of the paragraphs to look like misaligned.

Related: #276

Typst code:

#let ccwd = 12pt

// Set the text width to exact 32 Chinese characters.
#set page("iso-b5", margin: (x: 50% - ccwd * 16))

#set text(
  lang: "zh",
  font: ("Source Han Serif"),
  size: ccwd,
  overhang: false,
)

#set par(justify: true, first-line-indent: 2em)

// stripe background
#place(
  top + left,
  repeat(
    stack(
      dir: ltr,
      rect(width: ccwd, height: 24em, stroke: none, fill: rgb("#7092be70")),
      rect(width: ccwd, height: 24em, stroke: none)
    )
  )
)

first paragraph no indent

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Use Case

This is a common case in CJK typesetting.

[Enter-tainer]

I guess this is expected and comes from the adjustability of these punctuations. Per clreq: https://www.w3.org/TR/clreq/#h-compression_of_punctuation_marks_at_line_start

For the case of first-line indent, if an opening bracket is set at the beginning of the first line of the paragraph, half a character space can be trimmed before the bracket.

cc @peng1999

[Enter-tainer]

This causes the start of the paragraphs to look like misaligned.

The unaligned behavior is indeed weird given that justify: true has been specified.

It seems that clrep doesn't specify the adjustment amount of punctuations appears at the beginning of the line https://www.w3.org/TR/clreq/#procedures_for_inter-character_spacing_reduction

[peng1999]

@balthild

I'm not sure if this is a bug or a feature that haven't been implemented.

This is likely a bug. See below.

@Enter-tainer

It seems that clrep doesn't specify the adjustment amount of punctuations appears at the beginning of the line

As I understand it, the adjustment described in section 3.1.6.2 and 3.1.6.3 do not contribute to "line adjustment" described in Section 3.5 Line Adjustment of CLREQ. The former adjustments can be thought as "default adjustments" and should always take place. But in typst, when I implement this feature, the two default adjustment is being implemented as part of general line adjustment and becoming optional depending on the line length. I think this is wrong.

CLREQ section 3.1.6.3 mentioned "can be trimmed". I think it has an all-or-nothing meaning. Maybe we can provide an option in typst.

After all, I think this issue is not hard to fix. I encourage anyone who interested in it to make a PR.

[PgBiel]

Does #3744 fix this?

[balthild]

No, but the result has changed a little.

Tested with the current main branch (1e645b3).

[peng1999]

This problem now solves for paragraph with no indentation, but still exists otherwise.