3-Column Multilingual Book with Spanning Headers

Hi @ibnabouna, I was intrigued by this problem and I've written a script idea called "Align Paragraphs". It's a slightly complicated set up, as you can imagine, but I'll attach a sample document that you can use to see what I've done. I've put a bit of documentation in the script itself, too.

UPDATE 2024-09-08: I have re-written this script from ground up, and i think it is now much easier to use, and better. See documentation in the script listing. And definitely try out the attached demo.indd.

- Mark

/**
 * @file Align Parallel Paragraphs.js
 *
 * Purpose: to align "marked" paragraphs across different stories.
 *
 * This is useful for situations with "parallel" text threads,
 * for example, a layout where each page has a column of English,
 * a column of Hebrew, and a column of Greek (the texts are
 * translations and semantically linked). It is desired that
 * every subheading line up across the different languages
 * to make comparison by the reader easy.
 *
 * To achieve this scenario, you would add a marker to every paragraph
 * that needed to line up, in this case to each subheading.
 *
 * Set up:
 *
 *   0. Your document must have:
 *      - "Alignment Marker" object style (suggest specify size and non-printing)
 *      - "Spacer" paragraph style (suggest with tiny 0.01 leading, 0 paragraph spacing)
 *
 *   1. Create a non-printing marker triangle and apply "Alignment Marker" object style.
 *      The top of the marker's geometric bounds will be the alignment point.
 *
 *   2. Copy/paste a marker at the start of every line you want to align.
 *      Note: every time you add a marker, you must add one for each story.
 *      The script will attempt to align these "sets" of markers, eg. if you have 3 stories
 *      it will align the first marker of each story, and then align the second marker, etc.
 *
 * Usage:
 *   1. Select multiple text frames, each having a separate story.
 *   2. Run script.
 *
 * Technical:
 *   - the spacing is done by adding a "spacer" paragraph (with "Spacer" paragraph style)
 *     directly before the paragraph to be aligned, and setting its `spaceAfter`
 *   - all "spacer" paragraphs will be removed every time the script is re-run.
 *   - `settings.reset: true` will remove spacer paragraphs, but perform no alignments.
 *
 * @author m1b
 * @version 2024-09-08
 * @discussion https://community.adobe.com/t5/indesign-discussions/3-column-multilingual-book-with-spanning-headers/td-p/12923098
 */
function main() {

    var settings = {
        markerStyleName: 'Alignment Marker',
        spacerParagraphStyleName: 'Spacer',
        reset: false,
    };

    if (
        0 == app.documents.length
        || app.activeDocument.selection.length < 2
    )
        return alert('Please select two or more text frames and try again.');

    app.scriptPreferences.measurementUnit = MeasurementUnits.POINTS;

    var doc = settings.doc = app.activeDocument,
        frames = doc.selection,
        stories = [],
        unique = {};

    // check that we have the styles we need
    var markerStyle = settings.doc.objectStyles.itemByName(settings.markerStyleName),
        spacerParagraphStyle = settings.doc.paragraphStyles.itemByName(settings.spacerParagraphStyleName);

    if (!markerStyle.isValid)
        return alert('Marker object style "' + settings.markerStyleName + '" is invalid.');

    if (!spacerParagraphStyle.isValid)
        return alert('Spacer paragraph style "' + settings.spacerParagraphStyleName + '" is invalid.');

    // collect the stories
    for (var i = 0; i < frames.length; i++) {

        if (
            !frames[i].hasOwnProperty('parentStory')
            || unique[frames[i].parentStory.id]
        )
            continue;

        stories.push(frames[i].parentStory)
        unique[frames[i].parentStory.id] = true;

    }

    // remove all spacer paragraphs
    app.findGrepPreferences = NothingEnum.NOTHING;
    app.changeGrepPreferences = NothingEnum.NOTHING;
    app.findGrepPreferences.appliedParagraphStyle = spacerParagraphStyle;

    for (var i = 0, found; i < stories.length; i++) {

        found = stories[i].findGrep();

        for (var j = found.length - 1; j >= 0; j--)
            found[j].remove();

        stories[i].recompose();

    }

    if (settings.reset)
        return;

    // collect markers from the stories
    var allMarkers = [];

    for (var i = 0, markers; i < stories.length; i++) {

        allMarkers[i] = [];
        markers = stories[i].polygons;

        markerLoop:
        for (var j = 0; j < markers.length; j++) {

            if (markers[j].appliedObjectStyle.name !== settings.markerStyleName)
                continue markerLoop;

            allMarkers[i].push(markers[j]);

        }

    }

    // get number of alignments
    var alignmentCount = Infinity;
    for (var i = 0; i < allMarkers.length; i++)
        alignmentCount = Math.min(alignmentCount, allMarkers[i].length);

    // collect "alignments", which are sets of markers, one from each story
    var alignments = [];
    for (var i = 0; i < alignmentCount; i++) {

        alignments[i] = [];

        for (var j = 0; j < stories.length; j++)
            alignments[i][j] = allMarkers[j].shift();

    }

    // process each alignment
    for (var i = 0, markers, pageIndex, height; i < alignments.length; i++) {

        markers = alignments[i];
        pageIndex = -Infinity;
        height = -Infinity;

        // calculate the alignment's page index and height
        for (var j = 0, marker; j < markers.length; j++) {

            marker = markers[j];
            page = marker.parentPage;

            if (!page)
                throw new Error('Marker "' + marker.toSpecifier() + '" is not on a Page.');

            pageIndex = Math.max(pageIndex, page.documentOffset);

            if (pageIndex === page.documentOffset)
                height = Math.max(height, marker.geometricBounds[0]);

        }

        // align each marker
        for (var j = 0; j < markers.length; j++)
            alignMarker(markers[j], pageIndex, height);

    }

    /**
     * Align `marker` to page
     * @version 2024-09-08
     * @param {Polygon} marker - an anchored page item.
     * @param {Number} pageIndex - the alignment page.
     * @param {Number} height - the alignment Y coordinate.
     * @param {Number} [tolerance] - if the alignment required is less than this, don't bother (default: 0.001 pts).
     */
    function alignMarker(marker, pageIndex, height, tolerance) {

        if (undefined == tolerance)
            tolerance = 0.001;

        // add page breaks until marker is on the right page
        while (pageIndex > marker.parentPage.documentOffset) {
            marker.parent.paragraphs[0].insertionPoints[0].contents = SpecialCharacters.PAGE_BREAK;
            marker.parent.parentStory.paragraphs.previousItem(marker.parent.paragraphs[0]).appliedParagraphStyle = spacerParagraphStyle;
            marker.parent.parentStory.recompose();
        }

        // the distance to the alignment height
        var dy = marker.geometricBounds[0] - height;

        if (Math.abs(dy) < tolerance)
            // already aligned
            return;

        var spacerParagraph = marker.parent.parentStory.paragraphs.previousItem(marker.parent.paragraphs[0]);

        if (
            undefined == spacerParagraph
            || !spacerParagraph.isValid
            || spacerParagraph.parentTextFrames[0] !== marker.parent.parentTextFrames[0]
            || spacerParagraph.appliedParagraphStyle !== spacerParagraphStyle
        ) {
            // create the spacer paragraph
            marker.parent.paragraphs[0].insertionPoints[0].contents = '\r';
            spacerParagraph = marker.parent.parentStory.paragraphs.previousItem(marker.parent.paragraphs[0]);
            spacerParagraph.appliedParagraphStyle = spacerParagraphStyle;
        }

        // after adding the spacer paragraph, recalculate this
        dy = marker.geometricBounds[0] - height;

        if (dy > 0)
            throw new Error('Marker "' + marker.toSpecifier() + '" cannot be aligned due to bad sorting!');

        spacerParagraph.spaceAfter -= dy;

    };

};
app.doScript(main, ScriptLanguage.JAVASCRIPT, undefined, UndoModes.ENTIRE_SCRIPT, 'Align Parallel Paragraphs');