Displayed maths in +proof collapse in some cases

[gfngfn]

Reported here: https://twitter.com/mr_konn/status/1522197421312593921?s=20&t=zNgPdOp1zqE9rwpBUGIDQw

[konn]

A bit context: I typeset the code below:

@require: stdjareport
@require: math
document (|
  title = {あああああ};
  author = {mr_konn};
  date = {2022/04/23};
|) '<
+chapter{あああああ}<
  +proof{
    ううううううう

    \eqn(${x \in \bigcup_n A_n \Longleftrightarrow \bigcup_n A_n \in \mathcal{U}});
  }
  >
>

which resulted in:

On the other hand, if we add extra paragraph after eqn, the math gets done right, like this:

@require: stdjareport
@require: math
document (|
  title = {あああああ};
  author = {mr_konn};
  date = {2022/04/23};
|) '<
+chapter{あああああ}<
  +proof{
    ううううううう

    \eqn(${x \in \bigcup_n A_n \Longleftrightarrow \bigcup_n A_n \in \mathcal{U}});

    ええええ
  }
  >
>

[sankantsu]

I did some experiment and noticed some points.

• \eqn uses embed-block-breakable, followed by omit-skip-after.
• +proof put inline-fil and some endmark character at the end of commend.

In the case where the \eqn is the last content of +proof command, it seems that the omit-skip-after at the end of \eqn command absorbs the inline-fil before the endmark character and it causes line breaking problem.

Here is a simplified reproduction of the original problem and some workarounds.

@require: stdjareport

let-block ctx +add-endmark it =
    let ib-box = read-inline ctx {▪} in
    line-break true true ctx
        (read-inline ctx it ++ inline-fil ++ ib-box)

% guard (null inline-frame)
let guard = 
    let pads = (0pt,0pt,0pt,0pt) in
    let dc _ _ _ _ = [] in
    inline-frame-inner pads dc inline-nil

let-inline ctx \guard = guard

let-block ctx +add-endmark-with-guard it =
    let ib-box = read-inline ctx {▪} in
    line-break true true ctx
        (read-inline ctx it ++ guard ++ inline-fil ++ ib-box) % put guard before inline-fil

let-inline ctx \embed-block it =
    let ib = read-inline ctx it in
    let bb = line-break true true ctx (inline-fil ++ ib ++ inline-fil) in
    inline-fil ++ embed-block-breakable ctx bb ++ omit-skip-after

let-inline ctx \embed-block-with-no-omit-skip it =
    let ib = read-inline ctx it in
    let bb = line-break true true ctx (inline-fil ++ ib ++ inline-fil) in
    inline-fil ++ embed-block-breakable ctx bb

in

document (|
  title = {\#326: Displayed math in \+proof collapse};
  author = {mr_konn};
|) '<
    +chapter {Simplified reproduction} <
        +section {Case 1 (no sentence after embed-block)} <
            +add-endmark {
                First sentence.
                \embed-block {collapsed}
            }
        >
        +section {Case 2 (extra sentence after embed-block)} <
            +add-endmark {
                First sentence.
                \embed-block {not collapsed}
                Second sentence.
            }
        >
        +section {Case 3 (remove omit-skip)} <
            +add-endmark {
                First sentence.
                \embed-block-with-no-omit-skip {manual guard after block}
            }
        >
        +section {Case 4 (manual guard)} <
            +add-endmark {
                First sentence.
                \embed-block {manual guard after block}
                \guard;
            }
        >
        +section {Case 5 (modify \+proof command)} <
            +add-endmark-with-guard {
                First sentence.
                \embed-block {guarded command}
            }
        >
    >
>

The first workaround (Case 3) is to remove omit-skip-after.

Another workaround (Case 4,5) is to add some guard before inline-fil and the endmark.
It seems that the inline-frame-inner with no content effectively works as guard for preventing omit-skip-after to absorb inline-fil.

I don't know the details about line-breaking implentation and do not know best solution for this problem, but I hope this workaround give some insight.

[gfngfn]

Good catch! And thank you for making a detailed comparison.

The second workaround looks better in my opinion; the following exemplifies why omit-skip-after is needed:

@require: stdjareport

let-inline ctx \display-with-skip-omission it =
  let ib = read-inline ctx it in
  let bb = line-break true true ctx (inline-fil ++ ib ++ inline-fil) in
  inline-fil ++ embed-block-breakable ctx bb ++ omit-skip-after

let-inline ctx \display-without-skip-omission it =
  let ib = read-inline ctx it in
  let bb = line-break true true ctx (inline-fil ++ ib ++ inline-fil) in
  inline-fil ++ embed-block-breakable ctx bb

let text =
  {
    The quick brown fox jumps over the lazy dog.
    The quick brown fox jumps over the lazy dog.
    The quick brown fox jumps over the lazy dog.
  }

in
document (|
  title = {Need for `omit-skip-after`};
  author = {`@gfngfn`};
|) '<
  +p{
    #text;

    \display-with-skip-omission{
      Texts in the embedded block
    }

    #text;
  }
  +p{
    #text;

    \display-without-skip-omission{
      Texts in the embedded block
    }

    #text;
  }
  +p{
    Take a careful look at the indentations after the embedded blocks above!
    You can nonetheless prevent indentations from being inserted simply by adding `%`:
  }
  +p{
    #text;

    \display-without-skip-omission{
      Texts in the embedded block
    }%

    #text;
  }
>