Adjusting the frame width on a page in flowing text

Hi @uniq1, here is a quick script for it. See if it works for you. First put your cursor somewhere in the paragraph where you want to split and run script.

- Mark

/**
 * @file Indent Split Text Frame.js
 *
 * Place cursor in text and script will split
 * into two text frames, the second one indented
 * up until the next margin column position.
 *
 * @author m1b
 * @version 2024-12-23
 * @discussion https://community.adobe.com/t5/indesign-discussions/adjusting-the-frame-width-on-a-page-in-flowing-text/m-p/15052834
 */
function main() {

    var settings = {
        addFrameBreakCharacter: false,
        overlap: 0,
    }

    app.scriptPreferences.measurementUnit = MeasurementUnits.POINTS;

    const OVERLAP = settings.overlap,
        TOLERANCE = 0.01;

    var doc = app.activeDocument,
        item = doc.selection[0];

    if (!item || !item.isValid)
        return alert('Please place text cursor into your text and try again.');

    if (
        !item.hasOwnProperty('paragraphs')
        || !item.hasOwnProperty('parentTextFrames')
    )
        return alert('Please place your cursor into text frame and try again.');

    var frame = item.parentTextFrames[0],
        oldFrameBounds = frame.geometricBounds.slice(),
        newFrameBounds = oldFrameBounds.slice(),
        page = frame.parentPage;

    if (!page.isValid)
        return alert('Text Frame is no on a page.');

    // the place where the frame will split
    var splitHeight = item.paragraphs[0].characters[0].baseline - item.paragraphs[0].characters[0].ascent;

    oldFrameBounds[2] = splitHeight + OVERLAP;
    newFrameBounds[0] = splitHeight;

    if (page.marginPreferences.columnsPositions.length <= 2)
        return alert('This page has no margin columns.');

    var x,
        i,
        columnFound;

    if (PageSideOptions.RIGHT_HAND === page.side) {

        // loop over the column positions, right to left
        for (i = page.marginPreferences.columnsPositions.length - 3; i >= 0; i = i - 2) {

            x = page.bounds[1]
                + page.marginPreferences.left
                + page.marginPreferences.columnsPositions[i];

            if (x + TOLERANCE < frame.geometricBounds[3]) {
                newFrameBounds[3] = x;
                columnFound = true;
                break;
            }
        }

    }

    else {
        // loop over the column positions, left to right
        for (i = 2; i <= page.marginPreferences.columnsPositions.length; i = i + 2) {

            x = page.bounds[1]
                + (doc.documentPreferences.facingPages ? page.marginPreferences.right : page.marginPreferences.left)
                + page.marginPreferences.columnsPositions[i];

            if (x - TOLERANCE > frame.geometricBounds[1]) {
                newFrameBounds[1] = x;
                columnFound = true;
                break;
            }
        }

    }

    if (!columnFound)
        // no columns available
        return;

    if (settings.addFrameBreakCharacter)
        // insert a frame break
        item.paragraphs[0].insertionPoints[0].contents = SpecialCharacters.FRAME_BREAK;

    var newFrame = page.textFrames.add({ geometricBounds: newFrameBounds });

    frame.geometricBounds = oldFrameBounds;
    frame.nextTextFrame = newFrame;

};
app.doScript(main, ScriptLanguage.JAVASCRIPT, undefined, UndoModes.ENTIRE_SCRIPT, 'Indent Split Text Frame');

Edit 2024-12-22: added handling of right-hand pages, as per comment below.

Edit 2024-12-23: fixed bug in handling left-hand facing page, as per comment below.