Hi Sébastien,
honestly the plan sounds too complicated to me. It is confusing to me, and I am watching the development fairly closely, I suspect that for other downstream users of the library it would be too much churn.
I propose to simply:
1) keep the stable gtk3 release in sync with the gnome versioning (3.22) with deprecations etc. The two minor API additions seems very minor features and I think they can wait for the next devel cycle, so in a few months we will release 3.24 with those additions as we usually do
2) Release 3.90 that uses gtk 3.90 and does the namespace change. The namespace change is pretty mechanical so it can be easily done at the same time of adopting gtk4 and having a stable gtk3 version with just the namespace change is overkill and the weird version number will confuse people. If needed we can make the first gtk 3.90.0 release just be the namespace change without bumping gtk and bump in gtk in 3.90.1 so that people doing the porting have a stepping stone. The ABI break between 3.90.0 and 3.90.1 would not be a problem since 3.90 series is not abi stable anyway.
