Userscript to persist "Add N more rows at bottom" value in Google Spreadsheets

GoogleDocsExtensions.userscript.js

// ==UserScript==
// @name        GoogleDocsExtensions
// @description Adds some features to Google Docs
// @match       https://docs.google.com/spreadsheets/d/*
// ==/UserScript==

var storage = window.localStorage;
var oldHref = document.location.href;
var bodyList = document.querySelector("body");
var currentHref = null;
var currentRowsInput = null;
    
var observer = new MutationObserver(function() {
    if (oldHref != document.location.href) {
        oldHref = document.location.href;
    }
    checkRowsInput();
});

var config = {
    childList: true,
    subtree: true
};

observer.observe(bodyList, config);

function checkRowsInput(){
    var rowsInput = document.querySelector('.addRowsInput');
    if (!rowsInput) return;
    if(rowsInput != currentRowsInput){
        currentRowsInput = rowsInput;
        if(!rowsInput.getAttribute("isSetChangeHandler")){
            rowsInput.setAttribute("isSetChangeHandler", true);
            rowsInput.addEventListener("change", function(){
                storage.setItem(oldHref, rowsInput.value);
            })
        }
        if(currentHref != oldHref){
            var rowsValue = storage.getItem(oldHref);
            if(rowsValue){
                rowsInput.value = rowsValue;
            }
            currentHref = oldHref;
        }
    }
}

Used in Safari with https://github.com/quoid/userscripts extension