Resize clipping mask to Artboard

Hi @StefanStehlik, I wrote a quick script that tries to do what you described. Please give it a try and let me know how it goes.

- Mark

/**
 * Resize Clipping Mask To Active Artboard Rect.
 */
(function () {

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

    resizeClippingMaskToActiveArtboard(item);


    /**
     * Resizes item's clipping mask
     * to match artboard bounds.
     * @author m1b
     * @version 2022-10-20
     * @param {GroupItem} item - an Illustrator GroupItem with clipping mask.
     * @returns {Boolean} - success.
     */
    function resizeClippingMaskToActiveArtboard(item) {

        if (
            item == undefined
            || !item.hasOwnProperty('clipped')
            || item.clipped !== true
        )
            // no clipping mask
            return;

        var mask;
        for (var i = 0; i < item.pageItems.length; i++) {
            if (item.pageItems[i].clipping == true) {
                mask = item.pageItems[i];
                break;
            }
        }

        if (mask == undefined)
            return false;

        // get item's parent document
        var doc = item;
        while (
            doc.hasOwnProperty('parent')
            && doc.constructor.name != 'Document'
        )
            doc = doc.parent;

        var ab = doc.artboards[doc.artboards.getActiveArtboardIndex()],
            bounds = ab.artboardRect;

        // set mask size to match artboard rect
        mask.position = [bounds[0], bounds[1]];
        mask.width = bounds[2] - bounds[0];
        mask.height = -(bounds[3] - bounds[1]);

    };

})();