Ok, so I think I figured out what's going on here.
Some of the sync engines in mobile (the newer ones, using the Rust components) aren't wired up to allow pointing to stage. We at least still do have a few older sync engines ("history" being the one I've confirmed so far) that does allow overwriting the server in iOS to point to stage. So...what I think needs to be tested here (:rfk, would love your input as well) is that you can Sync across Desktop <-> say, iOS using say, History.
I've confirmed that syncing History DOES work against staging in iOS (while say "bookmarks" do not, which would be explained by the fact that they're using Rust components in iOS).
Bug 1589565 Comment 16 Edit History
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Ok, so I think I figured out what's going on here.
Some of the sync engines in mobile (the newer ones, using the Rust components) aren't wired up to allow pointing to stage. We at least still do have a few older sync engines ("history" being the one I've confirmed so far) that does allow overwriting the server in iOS to point to stage. So...what I think needs to be tested here (:rfk, would love your input as well) is that you can Sync across Desktop <-> say, iOS using say, History.
I've confirmed that syncing History DOES work against staging in iOS after changing FxA password (while say "bookmarks" do not, which would be explained by the fact that they're using Rust components in iOS).