A function for ArcGIS Arcade to break a string into lines approximately matching a target length.

arcade_line_length_enforcer.js

// Line-break enforcer, for instances where ArcGIS Pro won't automatically
// insert line breaks in long strings (e.g. in composite callouts).
// The targetWidth parameter is a character limit that the function will
// use to break the input str into multiple lines. Individual lines may
// be slightly longer or shorter than the targetWidth, depending on where
// space characters exist that can be replaced with line breaks.
function insertNewLines(str, targetWidth) {
    var words = Split(str, " ")
    var lines = []
    var curLine = ""
    if (Count(words) <= 1) { return str }  // str does not contain distinct words
    for (var i in words) {
        var word = words[i]
        var wordLength = Count(word) + 1  // Add one to account for the required space character
        var curLength = Count(curLine)
        if (curLength + wordLength < targetWidth) {
            // Add current word to current line if combined length below target
            curLine = Concatenate([curLine, word], " ")
        } else if (Abs(targetWidth - curLength) < Abs(targetWidth - (curLength + wordLength))) {
            // If current line's length is closer to target without new word, begin a new line
            Push(lines, curLine)
            curLine = word
        } else {
            // Add new word to current line and then begin a new line
            curLine = Concatenate([curLine, word], " ")
            Push(lines, curLine)
            curLine = ""
        }
    }
    // Push final line if not blank
    if (!IsEmpty(curLine)) {
        Push(lines, curLine)
    }
    // Join individual lines with newline character
    return Concatenate(lines, TextFormatting.NewLine)
}