How to grab all checkboxes in GMail?

I am trying to just report back all checkboxes in Gmail for a small background script. Eventually i want to return only those that are checked. The checkboxes that are beside each email, used to select them and operate them, are indicated by role=”checkbox”.

With current code, i only get an empty list, and i can’t figure out why. Here is my code:

background.js

console.log("Start");
browser.contextMenus.create(
    {
        id: "AlertTest",
        title: "Test2",
        contexts: ["all"],
    },
    () => void browser.runtime.lastError,
);

browser.contextMenus.onClicked.addListener((info, tab) => {
    switch (info.menuItemId) {
    case "AlertTest":
        //console.log(info.selectionText);
        var ele = document.querySelectorAll("[role=checkbox]");
        console.log(ele);
        break;
    }
});

function onError(error) {
    console.log(error);
}