print.trunc.cols shows nothing if first column is too long

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@venom1204 is already working on this.

Since Apr 24, 2026.

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Description

beginner-task print

related to #7716

> options(datatable.print.trunc.cols=TRUE)
> options(width=10)
> data.table(x="1234567", y=1, z=2, a=3)
4 variables not shown: [x, y, z, a]
> data.table(y=1, z=2, a=3, x="1234567")
   y z a
1: 1 2 3
1 variable not shown: [x]

we see that the first table has 4 variables not shown, whereas the second has only 1.
the tables have the same contents but different column order.
we could imagine improving the print method so that it only hides x (and prints the others) in both cases.

for reference ?print.data.table says

trunc.cols: If ‘TRUE’, only the columns that can be printed in the
          console without wrapping the columns to new lines will be
          printed (similar to ‘tibbles’).
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