diff --git a/pkg/spec/src/runtime-entry.ts b/pkg/spec/src/runtime-entry.ts
index 23114d9..c1f0f87 100644
--- a/pkg/spec/src/runtime-entry.ts
+++ b/pkg/spec/src/runtime-entry.ts
@@ -29,6 +29,7 @@ export type SerializedAction =
       toX: number;
       toY: number;
       durationMillis: number;
+      selector?: string;
     }
   | { kind: "PressKey"; key: string }
   | { kind: "Wait"; durationMillis: number };
@@ -90,20 +91,27 @@ export function serializeAction(action: ActionDescriptor | null): SerializedActi
       };
     }
     case "Scroll": {
-      const from = pointOf(action.from) ?? pointOf(action.in);
-      if (!from) return null;
-      // The builtin generator pre-computes `to`; an author Scroll without it
-      // collapses to a zero-length gesture the runner re-derives from bounds.
-      const to = pointOf(action.to) ?? from;
-      return {
+      // The builtin generator pre-computes the whole gesture. An author names
+      // the container instead and leaves the drag to the runner, which sizes it
+      // from that container's bounds: sending the container's own point as both
+      // endpoints would be a drag from a point to itself, which the runner
+      // executes as written, and sending no container at all would scroll
+      // whatever else is on screen.
+      const from = pointOf(action.from);
+      const to = pointOf(action.to);
+      const gesture =
+        from && to
+          ? { fromX: from.x, fromY: from.y, toX: to.x, toY: to.y }
+          : { fromX: 0, fromY: 0, toX: 0, toY: 0 };
+      const out: SerializedAction = {
         kind: "Scroll",
         direction: action.direction,
-        fromX: from.x,
-        fromY: from.y,
-        toX: to.x,
-        toY: to.y,
+        ...gesture,
         durationMillis: DEFAULT_SWIPE_DURATION,
       };
+      const container = pointOf(action.in);
+      if (container?.selector) out.selector = container.selector;
+      return out;
     }
     case "PressKey":
       return { kind: "PressKey", key: action.key };
diff --git a/pkg/spec/test/runtime-entry.test.ts b/pkg/spec/test/runtime-entry.test.ts
index d80ca2c..6a36473 100644
--- a/pkg/spec/test/runtime-entry.test.ts
+++ b/pkg/spec/test/runtime-entry.test.ts
@@ -75,14 +75,60 @@ test("Swipe defaults durationMillis to 250", () => {
   assert.equal(action.durationMillis, 250);
 });
 
-test("Scroll emits direction plus from/to point and duration", () => {
+// An author Scroll names a container and leaves the drag to the runner, which
+// sizes it from that container's bounds. Sending the container's own point as
+// both endpoints would be a drag from a point to itself, which the runner takes
+// at face value and dispatches as a zero-length gesture.
+test("an author Scroll carries no endpoints", () => {
   assert.deepEqual(serializeAction({ kind: "Scroll", direction: "down", in: POINT }), {
     kind: "Scroll",
     direction: "down",
-    fromX: 12,
-    fromY: 34,
-    toX: 12,
-    toY: 34,
+    fromX: 0,
+    fromY: 0,
+    toX: 0,
+    toY: 0,
+    durationMillis: 250,
+  });
+});
+
+test("an author Scroll names the container it scrolls", () => {
+  assert.deepEqual(serializeAction({ kind: "Scroll", direction: "down", in: "id:list" }), {
+    kind: "Scroll",
+    direction: "down",
+    fromX: 0,
+    fromY: 0,
+    toX: 0,
+    toY: 0,
+    durationMillis: 250,
+    selector: "id:list",
+  });
+  assert.deepEqual(
+    serializeAction({
+      kind: "Scroll",
+      direction: "down",
+      in: { x: 1, y: 2, selector: "id:list" },
+    }),
+    {
+      kind: "Scroll",
+      direction: "down",
+      fromX: 0,
+      fromY: 0,
+      toX: 0,
+      toY: 0,
+      durationMillis: 250,
+      selector: "id:list",
+    },
+  );
+});
+
+test("a Scroll with no container scrolls the screen rather than dropping", () => {
+  assert.deepEqual(serializeAction({ kind: "Scroll", direction: "up" }), {
+    kind: "Scroll",
+    direction: "up",
+    fromX: 0,
+    fromY: 0,
+    toX: 0,
+    toY: 0,
     durationMillis: 250,
   });
 });