Draft but tips/guidance is quite welcome if there is any

Functionality-wise it's basically ready and works really well for me, I'll just try to polish the code a bit more before the review.

Edit:
Or not really ready. Hiding the scrollbar via traditional methods* makes the animation slow down on mouse events for some reason. Still stuck on this.

* "traditional methods" being:

    let scrolled = ScrolledWindow::builder()
        .vscrollbar_policy(gtk::PolicyType::Never)
        .hscrollbar_policy(gtk::PolicyType::External)
        .build();

    if let Some(hbar) = scrolled.hscrollbar() {
        hbar.hide(); // or hbar.set_visible(false)
    }

Just a thought if you're still struggling with this. A radically different approach, but it would probably be relatively trivial to use Cairo and Pango for this. You could draw text onto a "label" canvas and just translate the text coords.

Thank you for the suggestion. I'll try this approach next, though definitely not anytime soon as I'll be busy for a while, so if anyone wants to pick this up that'd be welcome.

Alternatively, we could just wait for the GTK4 port to be completed, and maybe GTK4 would have a more sensible behavior wrt to that one last issue of hiding the scrollbar in a theming-independent way and without causing slowdown on mouse movement.

Superseded by #1250.

GTK4 actually fixes that scrollbars bug, so pretty much no changes aside from updating to GTK4 and expanding what's configurable.