Mostly, I've never liked split-screen UIs and would much prefer a Gmail-like interface, like in SnappyMail:

Similarly, I've never understood why Nextcloud Mail (and also now Cloudron Mail) default to having such a tiny compose pop-up:

Again, I much prefer being able to use all of my screens real estate, again, like in SnappyMail:

Perhaps one way to achieve this would be to remember that last size it was resized to? then I could just resize it to e.g. something like this and it'd remember that's how I like it?

Also, it's a strange UX that when minimising the compose window it just become as random little floating bar, rather than e.g. being docked down the bottom of the screen as in Gmail:

@timconsidine said:

show read mail in different colour, not simply a non-bold same colour as unread mail

I've been using this code with the Stylus plugin locally in Firefox to fix this same issue on Roundcube (class name is different though). This one is using the proper class for the CR mail app. It adds a pulsating green dot (customizable) for all unread mail.

 .mail-item.unread::before {
    content: "●"; 
    color:#0dff00;
    font-size: 16px;
    animation: pulseUnread 1s infinite;
}

@keyframes pulseUnread {
    0%   { opacity: 0.2; }
    50%  { opacity: 1; }
    100% { opacity: 0.2; }
}

Preview