#target illustrator

var doc = app.activeDocument;
var sel = doc.selection;

// Recursive function to collect text frames
function getTextFrames(items, result) {
    for (var i = 0; i < items.length; i++) {
        var item = items[i];

        if (item.typename === "TextFrame") {
            result.push(item);
        } else if (item.typename === "GroupItem") {
            getTextFrames(item.pageItems, result);
        }
    }
}

var frames = [];
getTextFrames(sel, frames);

if (frames.length === 0) {
    alert("No text frames found.");
} else {
    var input = prompt("Paste your text (use returns to split):", "");

    if (input) {
        var parts = input.split(/\r\n|\r|\n/);

        // SORT frames: top-to-bottom, then left-to-right
        frames.sort(function(a, b) {
            var ay = a.top;
            var by = b.top;

            // If roughly same row, sort left to right
            if (Math.abs(ay - by) < 5) {
                return a.left - b.left;
            }

            // Otherwise sort top to bottom
            return by - ay;
        });

        // Fill in order
        for (var i = 0; i < frames.length; i++) {
            frames[i].contents = parts[i] || "";
        }
    }
}