print.trunc.cols off by one? #7716

Is this an off by one bug?

> options(datatable.print.trunc.cols=TRUE)
> options(width=10)
> (dt7=data.table(x="1234567"))
1 variable not shown: [x]
> (dt6=data.table(x="123456"))
        x
1: 123456

Above we see dt6 is 9 characters wide (6 characters of data and 3 of row number, colon, space), and dt7 gets truncated, even though I expected that it should print (we have enough space for it with width=10).

Below we see that dt7 prints if we set width=11, which enables printing of this 10 character wide table.

> options(width=11)
> dt7
         x
1: 1234567

Does the current code count the trailing newline as one of the characters toward the width limit? (I expect it should not)

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’).

?options says

     ‘width’:
          controls the maximum number of columns on a line used in