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I wonder why SideEffect <https://developer.android.com/develop/ui/compose/side-effects|documentation page> doesn't mention what thread coroutines will be launched on. Will it be main thread always? Or render thread?

I need to know that when invoking side-effecting misc callbacks in effects.

Do you mean `LaunchedEffect`? Technically it can be configured when `Recomposer` is created, you can provide `effectCoroutineContext` in constructor. The rest of the effects are invoked on the same thread as composition (`setContent`) and the main clock.
Practically, it is the main thread by default.