I thought that the "yes" and "nop" suffixes were related to "page=yes" and "page=no" for a few reasons. First, because I thought "nop" could be shorthand for "no page." More importantly, however, I thought they might be related because setting page=no in \definepagecolumns does not appear to have any effect, and patching \page_col_stop_yes to do what \page_col_stop_nop does had the desired effect. But as you have shown, doing this causes problems with the transition back to a single-column layout, so I suppose that the \page_col_stop_yes and \page_col_stop_nop macros are meant to implement behavior dependent on the number of columns. Your suggestion to add a column if the column at the end of the environment is not the last one is elegant and works on different variations of the MWE, including cases with more than two columns.

Joey

On Sat, May 16, 2020 at 4:12 AM Wolfgang Schuster <wolfgang.schuster.lists@gmail.com> wrote:
Joey McCollum schrieb am 16.05.2020 um 05:45:
> All right, I've found a tentative solution that appears to work in all
> cases, but I'd like to know more about why the code I'm patching was
> implemented in the first place, because I don't want to break anything
> else. According to the code in page-pcl.mkiv, the \stoppagecolumns macro
> has two implementations, \page_col_stop_yes and \page_col_stop_nop. The
> choice of which implementation is used is determined by which of two
> implementations of \startpagecolumns (\page_col_start_yes
> and \page_col_start_nop) is used. That choice, in turn, is made in
> the \page_col_start macro, in the following lines:
>
> ```
> \c_page_col_n_of_columns\pagecolumnsparameter\c!n\relax
>     \ifnum\c_page_col_n_of_columns>\plusone
>       \expandafter\page_col_start_yes
>     \else
>       \expandafter\page_col_start_nop
>     \fi
> ```
>
> so if the number of columns (i.e., the n option for the
> pagecolumns environment) is more than one, the "yes" implementation of
> the start and stop commands is used.
>
> But this seems wrong. The naming convention for the two implementations
> ("yes" and "nop") suggests that they should be related the parameters
> "page=yes" and "page=no" (hence, no page, which is what I think "nop"
> abbreviates), but the choice of which implementation to use is
> determined not by the page parameter, but by the n parameter. Is this an
> error in the code, or am I just understanding this incorrectly? Is there
> some reason why a layout with more than one column would need special
> instructions to add a blank page after stopping the environment?

Why do you think "yes" and "nop" are related to the page-key?

> In any event, I was able to solve the problem by patching the
> \page_col_start_yes macro to invoke the \page_col_start_nop macro as
> follows:
>
> ```
>
> \unprotect
>
> \def\page_col_stop_yes
> {\page_col_stop_nop}
>
> \protect
>
> ```
>
> With this fix, the MWE I provided works regardless of which column the
> text ends in. I'm content to consider the problem solved, but I would
> certainly appreciate any feedback on my thoughts and questions about
> page-pcl.mkiv.

You broke now now normal single columns text after the pagecolumns
environment. A better fix is to add a check for the \column command at
the end of the environment and add it only when we aren't in the last
column.

\unexpanded\def\page_col_stop_yes
   {%\column % \page_otr_eject_page
    \ifnum\c_page_col_current<\c_page_col_n_of_columns
      \column
    \fi
    \page
    \endgroup
  % \setupoutputroutine[\s!singlecolumn]%
    \page_otr_command_set_vsize
    \page_otr_command_set_hsize
    \page
    \endgroup}

Wolfgang