Re-formats the angle bracket email quoting from a mailing list as actual indentation levels with nice lines. For use with TamperMonkey.

Swift Evolution Mailing List Formatter.js

// ==UserScript==
// @name         Swift-Evolution Email Formatting
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Formats the nested `>` indents of the mailing list into depth-marking lines
// @author       TheCodeWarrior
// @match        https://lists.swift.org/pipermail/swift-evolution/*
// @grant        none
// @require https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js
// ==/UserScript==

(function() {
    'use strict';

    // The list of selectors that find all the email root elements.
    let email_selectors = ['body > pre'];
    // This should match any unwanted separators between the `>`s and the lines. If the separator is part of the `>`'s text node this isn't required. To disable set this to `null`
    let line_prefix_removal = /^ /;

    function addGlobalStyle(css) {
        var head, style;
        head = document.getElementsByTagName('head')[0];
        if (!head) { return; }
        style = document.createElement('style');
        style.type = 'text/css';
        style.innerHTML = css;
        head.appendChild(style);
    }

    $(document).ready(function() {
        addGlobalStyle(`
.line-indent-wrap > i {
    display: inline-block;
}
.line-indent {
    display: inline-block;
    margin-left: 1.5em;
    border-left: 1px solid gray;
    padding-left: 1em;
}
`);

        function removeFirstSpace(jq) {
            if(line_prefix_removal !== null) {
            var elems = jq.contents();
            if(elems.length > 0) {
                var first = elems[0];
                if(first.nodeType === 3) {
                    first.nodeValue = first.nodeValue.replace(line_prefix_removal, "");
                }
                if(first.nodeType === 1) {
                    removeFirstSpace($(first));
                }
            }
            }
        }

        var doIt = true;
        var lastTag = null;
        $(email_selectors.join(', ')).contents().each(function () {
            if (this.nodeType === 3) {
                if(doIt) {
                    var newTag = $('<div class="line-indent-wrap"></div>');
                    this.parentNode.insertBefore(newTag[0], this);
                    this.parentNode.removeChild(this);
                    lastTag = newTag;
                    var currentText = $.trim($(this).text());
                    currentText.replace(/^\s*>*/, function(result) {
                        var count = $.trim(result).length;
                        for (var i = 0; i < count; i++) {
                            var tag = $('<span class="line-indent"></span>');
                            lastTag.append(tag);
                            lastTag = tag;
                        }
                    });
                    if(lastTag === newTag) {
                        newTag.append($(this));
                        doIt = currentText.endsWith("\n");
                    }
                } else {
                    lastTag.append($(this));
                    doIt = $(this).text().endsWith("\n");
                }
            }
            if (this.nodeType === 1) {
                var jq = $(this);
                doIt = jq.text().endsWith("\n");
                if(lastTag !== null) {
                    jq.remove();
                    removeFirstSpace(jq);
                    lastTag.append(jq);
                }
            }
        });

    });
})();