An incomplete shard snapshot will prevent a shard from moving away from a node (see `SnapshotInProgressAllocationDecider`). That means that when a shard snapshot completes it may become possible to perform some shard movements that previously were not possible, so we should trigger a reroute to start those movements. AFAICT we do not trigger such a reroute today.
server/src/test/java/org/elasticsearch/snapshots/SnapshotResiliencyTests.java
IndicesShardStoresAction.INSTANCE,
                    new TransportIndicesShardStoresAction(
                        transportService,
                        clusterService,
                        threadPool,
                        actionFilters,
                        indexNameExpressionResolver,
                        client
                    )
                );
                actions.put(
                    TransportNodesListGatewayStartedShards.TYPE,
                    new TransportNodesListGatewayStartedShards(
                        settings,
                        threadPool,
                        clusterService,
                        transportService,
                        actionFilters,
                        nodeEnv,
                        indicesService,
                        namedXContentRegistry
                    )
                );
                actions.put(
                    DeleteSnapshotAction.INSTANCE,
                    new TransportDeleteSnapshotAction(
                        transportService,
                        clusterService,
                        threadPool,
                        snapshotsService,