Hi! Thanks for the patch.

I think a better way may be to check the current URL perhaps? For example, when viewing a mail,
it looks something like this: https://www.fastmail.com/mail/Inbox/{mail-id}, but when replying to one, it's https://www.fastmail.com/mail/Inbox/compose/{mail-id}. Would this give you enough safety checks to know when to run the shortcuts?

Another idea is to somehow override the Fastmail bindings. In their source code they have this inside mail.mod.js:

    Uo = {
        r: "reply",
        a: "replyAll",
        "Alt-r": "replySender",
        f: "forward",
        n: "goNext",
        p: "goPrevious",
        e: "toggleExpand",
        s: "toggleFlagged",
        '"': "toggleQuotes",
        ":": "toggleDetails",
        ";": "showActions",
        "Shift-E": "expandAll",
        "Alt-Shift-E": "collapseAll",
        "Shift-L": "showRemoteImages",
        "Cmd-a": "selectMessageBody"
    },

perhaps one could extend it and simply add the arrow shortcuts there. But I think the URL approach is simpler