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Create a hierarchy of branches/PRs: z > y > x. Merge y. Gitlab will automatically retarget z to x; GitHub will orphan z.

I do wish Gitlab would spend some time better organizing their settings menus, but aside from that it seems to be more complete.

I'm not sure when it changed (sometime this year?) but GH automatically changes the PR base now.

I do stacked PRs for complex changes, and Github has been updating branch bases to support this workflow for at least two years now.

There are a few free open source tools available these days to manage the stacked PRs. Eg: https://github.com/aviator-co/av