long column names go past width

[tdhock]

Related to #7788 which implemented this abbreviation:

> options(width=20);data.table(x="abcdefghijklmnopqrstuvwxyz")
                   x
              <char>
1: abcdefghijklmn...

long column names can still go over width:

> options(width=20);data.table(abcdefghijklmnopqrstuvwxyz=1)
   abcdefghijklmnopqrstuvwxyz
                        <num>
1:                          1
> options(width=20, datatable.print.trunc.cols=TRUE);data.table(abcdefghijklmnopqrstuvwxyz=1)
1 variable not shown: [abcdefghijklmnopqrstuvwxyz <num>]

I would expect some output like:

> options(width=20);data.table(abcdefghijklmnopqrstuvwxyz=1)
   abcdefghijklmnop…
               <num>
1:                 1
> options(width=20, datatable.print.trunc.cols=TRUE);data.table(abcdefghijklmnopqrstuvwxyz=1)
1 variable not shown:
abcdefghijklm… <num>

in comparison, others go over width too

> options(width=20);data.frame(x="abcdefghijklmnopqrstuvwxyz")
                           x
1 abcdefghijklmnopqrstuvwxyz
> options(width=20);data.frame(abcdefghijklmnopqrstuvwxyz=1)
  abcdefghijklmnopqrstuvwxyz
1                          1
> options(width=20);tibble::tibble(abcdefghijklmnopqrstuvwxyz=1)
# A tibble: 1 × 1
# ℹ 1 more
#   variable:
#   abcdefghijklmnopqrstuvwxyz <dbl>
> options(width=80);tibble::tibble(abcdefghijklmnopqrstuvwxyz=1)
# A tibble: 1 × 1
  abcdefghijklmnopqrstuvwxyz
                       <dbl>
1                          1
> 

this is not high priority because long column names are not frequent.

[venom1204]

hi @tdhock @joshhwuu

I looked into issue and have a proposed fix. It applies header truncation using the existing char.trunc() logic, updates empty data.table summaries and the variables not shown footer, and integrates the truncated widths into the layout calculations.

if this sounds good i will be happy to proceed with teh pr