508120 - Bad line breaks within a mathematical expression

Status: RESOLVED DUPLICATE of bug 534962

(Core :: MathML, defect)

Description

[Matthew Cargo]

User-Agent:       Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.12) Gecko/2009070811 Ubuntu/9.04 (jaunty) Firefox/3.0.12
Build Identifier: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.12) Gecko/2009070811 Ubuntu/9.04 (jaunty) Firefox/3.0.12

Firefox has nonstandard rules for choosing line breaks within a mathematical
expression. For example, in the expression
<math 
  xmlns="http://www.w3.org/1998/Math/MathML" display="inline" ><mi 
 >K</mi><mo 
 class="MathClass-open">(</mo><mo 
 class="MathClass-open">(</mo><msub><mrow 
 ><mi 
 >X</mi></mrow><mrow 
 ><mn>1</mn></mrow></msub 
 ><mo 
 class="MathClass-punc">,</mo><mo 
 class="MathClass-op">&#x2026;</mo><mo 
 class="MathClass-punc">,</mo><msub><mrow 
 ><mi 
 >X</mi></mrow><mrow 
 ><mi 
 >n</mi></mrow></msub 
 ><mo 
 class="MathClass-close">)</mo><mo 
 class="MathClass-close">)</mo></math>
it will allow a line break before the two closing parentheses. This looks terrible and is contrary to (say) TeX's rules for breaking inline math.
(Screenshot available on request.)

Reproducible: Always

Comment 1

[Frédéric Wang (:fredw)]

> I would emphasize that the desired solution to this is _not_ to 
> add additional tags in the mathml such as nobreak. Mathml is supposed 
> to describe the math content, not how it is laid out.  It is the 
> browser's job to render it correctly. I assume this is possible in 
> the cases above, because TeX is perfectly capable of choosing the 
> correct breaks in (say) $(x+y)+z=x+(y+z)$ (with much less verbosity).

I haven't really studied this yet, but here is what has been introduced in MathML3:

http://www.w3.org/TR/MathML3/chapter3.html#presm.linebreaking

Comment 2

[Matthew Cargo]

Thanks for clarifying the difference between these two bug reports.

Let me say a few things based on my experience typesetting
papers in TeX for PJM.  The vast majority of the time,
TeX makes the correct call: it allows line breaks just 
after binary operators like + and \times, and after relational 
operators like "=". It doesn't allow breaks after commas or
open parentheses.
  
All of this information is encoded in mathml (mathopen, mathbin etc), so it
should be fully possible to replicate TeX's algorthm. 
That is, unless, as mentioned elsewhere, inline mathml and surrounding 
text is dealt with in completely separate parts of the code.

There are cases when one wishes explicitly disallow a line break,
by using \nobreak after (say) +, usually because
we'd prefer to break after something higher in the
expression's hierarchy. For example a(x+y)=(x+y)a, should break,
if at all, after the =, not the pluses.

But enforcing additional constraints is a secondary problem: Just like
TeX, a good mathml renderer should aim to get the line breaks
mostly right the first time.  One shouldn't have to manually specify
nobreak where there should never be breaks in the first place.

Comment 3

[Frédéric Wang (:fredw)]

The solution to the problem here is essentially to implement MathML 3 linebreaking. So I resolve this bug as duplicate.