Hi Rémi,
Sorry, it seems my last email didn't get sent for some reason.
Anyway, I wanted to say I did a quick test on Windows 10. Setting that registry value will make all stacks be allocated from top-down, even the main thread's stack (i.e. far above 4 GB).
So it seems that Windows doesn't really treat the stack special here. The fact it's allocated below 2G for most apps is due to the side-effect of the bottom-up allocation behavior on Windows by default.
The current Wine code probably does that since some apps depend on it, but IMO correct fix would be to have all allocations bottom-up like the staging patchset, since other apps depend on that.