Javascript - onload event

当我打开新的选项卡而不切换到它们时，我遇到了onload事件的问题。

我正试图为Chrome编写一个用户脚本，在pre标签后添加"复制"按钮。当我试图加载的页面是焦点时，它工作正常，但当我在后台打开一个新选项卡时，它不工作。

这个行为是预期的吗?

下面是我的代码:

// ==UserScript==
// @author       Vamsi Krishna | vamsi_ism@outlook.com
// @description  Adds a copy button after 'pre' tags.
// @grant        none
// @homepage     https://github.com/krikx/CopyCode
// @match        *://*/*
// @name         CopyCode
// @version      0.1
// ==/UserScript==
var css = "/* google fonts */ @import url(https://fonts.googleapis.com/css?family=Open+Sans); .kxbt { border-radius: 4px; cursor: pointer; display: inline-block; font-family: 'Open Sans', 'sans-serif'; font-size: 15px; margin-bottom: 5px; padding: 5px; padding-top: 3px; transition: .2s; } .blue { color: #55acee; border: 1px #55acee solid; } .blue:hover { background-color: #55acee; color: #fff; } .green { color: #2ecc71; border: 1px #2ecc71 solid; } .green:hover { color: #fff; background-color: #2ecc71; } .red { color: #e74c3c; border: 1px #e74c3c solid; } .red:hover { color: #fff; background-color: #e74c3c; } /* green and red are almost never used as timeout is too fast to observe any noticible difference */";
var style = document.createElement('style');
style.innerHTML = css;
document.getElementsByTagName('head')[0].appendChild(style);
function Init(){
    var blocks = document.getElementsByTagName('pre');
    for (var id = 0; id < blocks.length; id++){
        var btn = document.createElement('div');
        btn.innerText = 'Copy';
        btn.setAttribute('class', 'kxbt blue');
        InsertAfter(blocks[id], btn);
    }
}
function InsertAfter(refNode, newNode){
    refNode.parentNode.insertBefore(newNode, refNode.nextSibling);
    WireClick(newNode, refNode); // add the click funtionality
}
function WireClick(btn, block){
    btn.addEventListener('click', function(){
        Copy(btn, block);
    });
}
function Copy(btn, block){
    var range = document.createRange();
    range.selectNode(block);
    var sel = getSelection();
    sel.removeAllRanges(); // TODO | removing selection without backup
    sel.addRange(range);
    var success = document.execCommand('copy');
    sel.removeAllRanges(); // possible TODO
    var msg = success ? 'Copied!' : 'Error!';
    var tcls = success ? 'kxbt green' : 'kxbt red';
    btn.innerText = msg;
    btn.setAttribute('class', tcls);
    setTimeout( // revert back to blue in 256ms
        function(btn){
            btn.innerText = 'Copy';
            btn.setAttribute('class', 'kxbt blue');
        },
        256, btn
    )
}
function main(){
    var body = document.getElementsByTagName('body')[0];
    // do not inject twice
    if (body.hasAttribute('data-kxbt-inited')){
        return;
    }
    body.setAttribute('data-kxbt-inited', true);
    Init();
}
// wait until ready
window.addEventListener('load', main);