fix: lagging code (#6441)

Fixes a bug where the node would not reorg to a chain with a lower
height, even if it had a higher difficulty

base_layer/core/src/base_node/state_machine_service/states/listening.rs

@@ -380,19 +380,26 @@ fn determine_sync_mode(
         }
 
         // This is to test the block propagation by delaying lagging.
-        if local_tip_height.saturating_add(blocks_behind_before_considered_lagging) > network_tip_height {
-            info!(
-                target: LOG_TARGET,
-                "While we are behind, we are still within {} blocks of them, so we are staying as listening and \
-                 waiting for the propagated blocks",
-                blocks_behind_before_considered_lagging
-            );
-            return SyncStatus::BehindButNotYetLagging {
-                local: local.clone(),
-                network: network.claimed_chain_metadata().clone(),
-                sync_peers: vec![network.clone().into()],
+        // If the config is 0, ignore this set.
+        if blocks_behind_before_considered_lagging > 0 {
+            // Otherwise, only wait when the tip is above us, otherwise
+            // chains with a lower height will never be reorged to.
+            if network_tip_height > local_tip_height &&
+                local_tip_height.saturating_add(blocks_behind_before_considered_lagging) > network_tip_height
+            {
+                info!(
+                    target: LOG_TARGET,
+                    "While we are behind, we are still within {} blocks of them, so we are staying as listening and \
+                     waiting for the propagated blocks",
+                    blocks_behind_before_considered_lagging
+                );
+                return SyncStatus::BehindButNotYetLagging {
+                    local: local.clone(),
+                    network: network.claimed_chain_metadata().clone(),
+                    sync_peers: vec![network.clone().into()],
+                };
             };
-        };
+        }
 
         debug!(
             target: LOG_TARGET,