background.js

// Register the message display script for all newly opened message tabs.
messenger.messageDisplayScripts.register({
    js: [{ file: "messageDisplay/message-content-script.js" }],
    css: [{ file: "messageDisplay/message-content-styles.css" }],
});

// Inject script and CSS in all already open message tabs.
let openTabs = await messenger.tabs.query();
let messageTabs = openTabs.filter(
    tab => ["mail", "messageDisplay"].includes(tab.type)
);
for (let messageTab of messageTabs) {
    browser.tabs.executeScript(messageTab.id, {
        file: "messageDisplay/message-content-script.js"
    })
    browser.tabs.insertCSS(messageTab.id, {
        file: "messageDisplay/message-content-styles.css"
    })
}

/**
 * Add a handler for the communication with other parts of the extension,
 * like our message display script.
 *
 * Note: It is best practice to always define a synchronous listener
 *       function for the runtime.onMessage event.
 *       If defined asynchronously, it will always return a Promise
 *       and therefore answer all messages, even if a different listener
 *       defined elsewhere is supposed to handle these.
 * 
 *       The listener should only return a Promise for messages it is
 *       actually supposed to handle.
 */
messenger.runtime.onMessage.addListener((message, sender, sendResponse) => {
    // Check what type of message we have received and invoke the appropriate
    // handler function.
    if (message && message.hasOwnProperty("command")) {
        return commandHandler(message, sender);
    }
    // Return false if the message was not handled by this listener.
    return false;
});

// The actual (asynchronous) handler for command messages.
async function commandHandler(message, sender) {
    // Get the message currently displayed in the sending tab, abort if
    // that failed.
    const messageHeader = await messenger.messageDisplay.getDisplayedMessage(
        sender.tab.id
    );

    if (!messageHeader) {
        return;
    }

    // Check for known commands.
    switch (message.command) {
        case "getBannerDetails":
            return { text: `Attenzione: questa mail urta la tua' sensibilita' di sveglione che mica ti fregano a te`};
            break;    
        default:
            break;
    }
}