I've always thought ‘multi-document interfaces’ as we used to call them are an anti-pattern. I have a perfectly good window manager; why does every app need its own incompatible, usually inferior window manager built in? (Mind you on mobile I very much don't have a perfectly good window manager, and indeed can't even open multiple instances of most apps…)
>why does every app need its own incompatible, usually inferior window manager built in? You answered your own question, because a lot of applications work across multiple platforms, and if you want to have control over the experience because you don't know what capacities the OS's window manager has you need to abstract it away.
But I take your point, if you want to target the lowest common denominator of window managers it makes some sense to do your own window management. Mind you you could just ship both a browser and a window manager…
I wonder to what extent the pattern of applications doing their own window management masks (and therefore perpetuates) the problem of inadequate window managers.