diff --git a/docker/frontend/src/ui/Filmstrip.tsx b/docker/frontend/src/ui/Filmstrip.tsx
index 3ad262f..ed28327 100644
--- a/docker/frontend/src/ui/Filmstrip.tsx
+++ b/docker/frontend/src/ui/Filmstrip.tsx
@@ -92,12 +92,18 @@ export const Filmstrip = memo(function Filmstrip({
   // and not `scroll-behavior: smooth` on the rail, which glides across the whole
   // roll: a scroll event a frame, and a fresh window asked for at each one, of
   // frames the reader never stops on. The rail is put where it is wanted.
-  useEffect(() => {
+  const centre = useCallback(() => {
     const box = rail.current;
     const el = box?.children[at] as HTMLElement | undefined;
     if (!box || !el) return;
     box.scrollLeft = el.offsetLeft - box.clientWidth / 2 + el.clientWidth / 2;
-  }, [at, frames]);
+  }, [at]);
+
+  useEffect(() => {
+    centre();
+    // The frames are in the dependency too: the frame in hand is a child of the
+    // rail, and until the folder arrives there is nothing to stand on.
+  }, [centre, frames]);
 
   // Scrolling and resizing both move what the rail can reach: the phone's rail
   // is a different window on the same roll, and the tiles it now needs are not
@@ -115,14 +121,23 @@ export const Filmstrip = memo(function Filmstrip({
         report();
       });
     };
+    // A narrower rail is not the wide one with less of the roll in it: it is a
+    // different number of frames to the pixel the reader was standing on, so the
+    // same scrollLeft is a different frame — the strip left the frame on the
+    // stage behind and drew the roll from somewhere else down it. It is put back
+    // on that frame first, and the window is read from where it then stands.
+    const resized = () => {
+      centre();
+      again();
+    };
     box.addEventListener('scroll', again, { passive: true });
-    const ro = new ResizeObserver(again);
+    const ro = new ResizeObserver(resized);
     ro.observe(box);
     return () => {
       box.removeEventListener('scroll', again);
       ro.disconnect();
     };
-  }, [report]);
+  }, [report, centre]);
 
   return (
     // A wheel has no sideways axis, so a plain tick over the roll walks it along.