diff --git a/piker/fsp/_engine.py b/piker/fsp/_engine.py
index 70e86481..afd6aa2c 100644
--- a/piker/fsp/_engine.py
+++ b/piker/fsp/_engine.py
@@ -269,21 +269,23 @@ async def cascade(
 
                 async for msg in stream:
 
-                    # respawn the compute task if the source
-                    # array has been updated such that we compute
-                    # new history from the (prepended) source.
-                    diff = src.index - dst.index
+                    def desynced(src: ShmArray, dst: ShmArray) -> bool:
+                        diff = src.index - dst.index
+                        len_diff = abs(len(src.array) - len(dst.array))
+                        return (
+                            # the source is likely backfilling and we must
+                            # sync history calculations
+                            len_diff > 2 or
 
-                    # new_len = len(src.array)
+                            # we aren't step synced to the source and may be
+                            # leading/lagging by a step
+                            diff > 1 or
+                            diff < 0
+                        )
 
                     async def poll_and_sync_to_step(tracker):
-                        diff = src.index - dst.index
-                        while True:
-                            if diff in (0, 1):
-                                break
-
+                        while desynced(src, dst):
                             tracker, index = await resync(tracker)
-                            diff = src.index - dst.index
                             # log.info(
                             #     '\n'.join((
                             #         f'history index after sync: {index}',
@@ -293,18 +295,10 @@ async def cascade(
 
                         return tracker, diff
 
-                    # log.debug(f'diff {diff}')
-
-                    if (
-                        # the source is likely backfilling and we must
-                        # sync history calculations
-                        abs(len(src.array) - len(dst.array)) > 0 or
-
-                        # we aren't step synced to the source and may be
-                        # leading/lagging by a step
-                        diff > 1 or
-                        diff < 0
-                    ):
+                    # respawn the compute task if the source
+                    # array has been updated such that we compute
+                    # new history from the (prepended) source.
+                    if desynced(src, dst):
                         tracker, diff = await poll_and_sync_to_step(tracker)
 
                         # skip adding a last bar since we should be