kotlinx-coroutines-core/native/src/CoroutineContext.kt [44:108]:
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internal actual inline fun <T> withCoroutineContext(context: CoroutineContext, countOrElement: Any?, block: () -> T): T {
    val oldValue = updateThreadContext(context, countOrElement)
    try {
        return block()
    } finally {
        restoreThreadContext(context, oldValue)
    }
}

internal actual inline fun <T> withContinuationContext(continuation: Continuation<*>, countOrElement: Any?, block: () -> T): T {
    val context = continuation.context
    val oldValue = updateThreadContext(context, countOrElement)
    val undispatchedCompletion = if (oldValue !== NO_THREAD_ELEMENTS) {
        // Only if some values were replaced we'll go to the slow path of figuring out where/how to restore them
        continuation.updateUndispatchedCompletion(context, oldValue)
    } else {
        null // fast path -- don't even try to find undispatchedCompletion as there's nothing to restore in the context
    }
    try {
        return block()
    } finally {
        if (undispatchedCompletion == null || undispatchedCompletion.clearThreadContext()) {
            restoreThreadContext(context, oldValue)
        }
    }
}

internal fun Continuation<*>.updateUndispatchedCompletion(context: CoroutineContext, oldValue: Any?): UndispatchedCoroutine<*>? {
    if (this !is CoroutineStackFrame) return null
    /*
     * Fast-path to detect whether we have undispatched coroutine at all in our stack.
     *
     * Implementation note.
     * If we ever find that stackwalking for thread-locals is way too slow, here is another idea:
     * 1) Store undispatched coroutine right in the `UndispatchedMarker` instance
     * 2) To avoid issues with cross-dispatch boundary, remove `UndispatchedMarker`
     *    from the context when creating dispatched coroutine in `withContext`.
     *    Another option is to "unmark it" instead of removing to save an allocation.
     *    Both options should work, but it requires more careful studying of the performance
     *    and, mostly, maintainability impact.
     */
    val potentiallyHasUndispatchedCoroutine = context[UndispatchedMarker] !== null
    if (!potentiallyHasUndispatchedCoroutine) return null
    val completion = undispatchedCompletion()
    completion?.saveThreadContext(context, oldValue)
    return completion
}

internal tailrec fun CoroutineStackFrame.undispatchedCompletion(): UndispatchedCoroutine<*>? {
    // Find direct completion of this continuation
    val completion: CoroutineStackFrame = when (this) {
        is DispatchedCoroutine<*> -> return null
        else -> callerFrame ?: return null // something else -- not supported
    }
    if (completion is UndispatchedCoroutine<*>) return completion // found UndispatchedCoroutine!
    return completion.undispatchedCompletion() // walk up the call stack with tail call
}

/**
 * Marker indicating that [UndispatchedCoroutine] exists somewhere up in the stack.
 * Used as a performance optimization to avoid stack walking where it is not necessary.
 */
private object UndispatchedMarker: CoroutineContext.Element, CoroutineContext.Key<UndispatchedMarker> {
    override val key: CoroutineContext.Key<*>
        get() = this
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kotlinx-coroutines-core/wasmJs/src/CoroutineContext.kt [26:90]:
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internal actual inline fun <T> withCoroutineContext(context: CoroutineContext, countOrElement: Any?, block: () -> T): T {
    val oldValue = updateThreadContext(context, countOrElement)
    try {
        return block()
    } finally {
        restoreThreadContext(context, oldValue)
    }
}

internal actual inline fun <T> withContinuationContext(continuation: Continuation<*>, countOrElement: Any?, block: () -> T): T {
    val context = continuation.context
    val oldValue = updateThreadContext(context, countOrElement)
    val undispatchedCompletion = if (oldValue !== NO_THREAD_ELEMENTS) {
        // Only if some values were replaced we'll go to the slow path of figuring out where/how to restore them
        continuation.updateUndispatchedCompletion(context, oldValue)
    } else {
        null // fast path -- don't even try to find undispatchedCompletion as there's nothing to restore in the context
    }
    try {
        return block()
    } finally {
        if (undispatchedCompletion == null || undispatchedCompletion.clearThreadContext()) {
            restoreThreadContext(context, oldValue)
        }
    }
}

internal fun Continuation<*>.updateUndispatchedCompletion(context: CoroutineContext, oldValue: Any?): UndispatchedCoroutine<*>? {
    if (this !is CoroutineStackFrame) return null
    /*
     * Fast-path to detect whether we have undispatched coroutine at all in our stack.
     *
     * Implementation note.
     * If we ever find that stackwalking for thread-locals is way too slow, here is another idea:
     * 1) Store undispatched coroutine right in the `UndispatchedMarker` instance
     * 2) To avoid issues with cross-dispatch boundary, remove `UndispatchedMarker`
     *    from the context when creating dispatched coroutine in `withContext`.
     *    Another option is to "unmark it" instead of removing to save an allocation.
     *    Both options should work, but it requires more careful studying of the performance
     *    and, mostly, maintainability impact.
     */
    val potentiallyHasUndispatchedCoroutine = context[UndispatchedMarker] !== null
    if (!potentiallyHasUndispatchedCoroutine) return null
    val completion = undispatchedCompletion()
    completion?.saveThreadContext(context, oldValue)
    return completion
}

internal tailrec fun CoroutineStackFrame.undispatchedCompletion(): UndispatchedCoroutine<*>? {
    // Find direct completion of this continuation
    val completion: CoroutineStackFrame = when (this) {
        is DispatchedCoroutine<*> -> return null
        else -> callerFrame ?: return null // something else -- not supported
    }
    if (completion is UndispatchedCoroutine<*>) return completion // found UndispatchedCoroutine!
    return completion.undispatchedCompletion() // walk up the call stack with tail call
}

/**
 * Marker indicating that [UndispatchedCoroutine] exists somewhere up in the stack.
 * Used as a performance optimization to avoid stack walking where it is not necessary.
 */
private object UndispatchedMarker: CoroutineContext.Element, CoroutineContext.Key<UndispatchedMarker> {
    override val key: CoroutineContext.Key<*>
        get() = this
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