Re: underbraces get progressively more misaligned (w/ minimal example)

[Hans Hagen <pragma@wxs.nl>]

On 7/4/2014 11:34 AM, Sanjoy Mahajan wrote:
> Wolfgang Schuster <schuster.wolfgang@gmail.com> writes:
>
>> It does also depend on the width of the text because when I use 48pt
>> in the example below the height of the brace is always the same but
>> with 49pt the height increases after the first page.
>
> An interesting bug.  That's also consistent with my experiments, in which
>
>    \underbrace{\hbox{yyyy yyyy}}_{\rho}
>
> triggers the problem, but
>
>    \underbrace{\hbox{yyyy}}_{\rho}
>
> does not.

Hi Sanjoy,

After a day of testing, tracing, wondering etc Luigi and I figured out 
what was the problem it tooks us lots of debug prints -). Indeed there 
is a bug in luatex. An extensible is made up of a sequence

char
skip 0pt- xpt
char
skip 0pt- xpt
char
skip 0pt- xpt
char

And the code that determines the height/depth took the dimensions of 
skips into the equation while in fact glue has no such dimensions 
(leaders that are a special kind of glue have dimensions). Not the first 
place one looks for such an issue. Probably a side effect of copying a 
function and patching part.

So, some random value (not entirely random as it depends on what mem 
area the glue node is allocated from and that is kind of stable in a 
run) was added which results in the effects you noticed.

Because it is stable within a run (same successive errors) it doesn't 
result in oscillating (too many runs due to differences in results) and 
this is probably why it went unnoticed so long. Interesting. (Normally 
the pseudo random ht/dp were small.)

So, once this fix moves upstream you can check your suspicious cases. 
Luigi will push it in our experimental branch and I suppose that Taco 
will then push it into trunk and after that you can compile or wait till 
the garden carries it.

Thanks for noticing!

Hans


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