diff --git a/src/hub/reducers.test.ts b/src/hub/reducers.test.ts
new file mode 100644
index 00000000..8c403ae9
--- /dev/null
+++ b/src/hub/reducers.test.ts
@@ -0,0 +1,141 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2021 The Pybricks Authors
+
+import { Action } from '../actions';
+import { statusReportEvent } from '../ble-pybricks-service/actions';
+import { Status, statusToFlag } from '../ble-pybricks-service/protocol';
+import { didDisconnect } from '../ble/actions';
+import {
+    didFailToFinishDownload,
+    didFinishDownload,
+    didProgressDownload,
+    didStartDownload,
+} from './actions';
+import reducers, { HubRuntimeState } from './reducers';
+
+type State = ReturnType<typeof reducers>;
+
+test('initial state', () => {
+    expect(reducers(undefined, {} as Action)).toMatchInlineSnapshot(`
+        Object {
+          "downloadProgress": null,
+          "runtime": "hub.runtime.disconnected",
+        }
+    `);
+});
+
+describe('runtime', () => {
+    test.each(Object.values(HubRuntimeState))('didDisconnect', (startingState) => {
+        // all states are overridden by disconnect
+        expect(
+            reducers({ runtime: startingState } as State, didDisconnect()).runtime,
+        ).toBe(HubRuntimeState.Disconnected);
+    });
+
+    test('didStartDownload', () => {
+        // download ignored if disconnected
+        expect(
+            reducers(
+                { runtime: HubRuntimeState.Disconnected } as State,
+                didStartDownload(),
+            ).runtime,
+        ).toBe(HubRuntimeState.Disconnected);
+        expect(
+            reducers({ runtime: HubRuntimeState.Idle } as State, didStartDownload())
+                .runtime,
+        ).toBe(HubRuntimeState.Loading);
+    });
+
+    test('didProgressDownload', () => {
+        // download ignored if disconnected
+        expect(
+            reducers(
+                { runtime: HubRuntimeState.Disconnected } as State,
+                didProgressDownload(0),
+            ).runtime,
+        ).toBe(HubRuntimeState.Disconnected);
+
+        // this shouldn't have any effect
+        expect(
+            reducers(
+                { runtime: HubRuntimeState.Loading } as State,
+                didProgressDownload(0),
+            ).runtime,
+        ).toBe(HubRuntimeState.Loading);
+    });
+
+    test('didFinishDownload', () => {
+        // download ignored if disconnected
+        expect(
+            reducers(
+                { runtime: HubRuntimeState.Disconnected } as State,
+                didFinishDownload(),
+            ).runtime,
+        ).toBe(HubRuntimeState.Disconnected);
+
+        // normal operation
+        expect(
+            reducers({ runtime: HubRuntimeState.Loading } as State, didFinishDownload())
+                .runtime,
+        ).toBe(HubRuntimeState.Loaded);
+    });
+
+    test('didFinishDownload', () => {
+        // download ignored if disconnected
+        expect(
+            reducers(
+                { runtime: HubRuntimeState.Disconnected } as State,
+                didFailToFinishDownload(),
+            ).runtime,
+        ).toBe(HubRuntimeState.Disconnected);
+
+        // normal operation for error path
+        expect(
+            reducers(
+                { runtime: HubRuntimeState.Loading } as State,
+                didFailToFinishDownload(),
+            ).runtime,
+        ).toBe(HubRuntimeState.Idle);
+    });
+
+    test('statusReportEvent', () => {
+        // don't ever expect this to happen in practice since we can't receive
+        // updates while disconnected
+        expect(
+            reducers(
+                { runtime: HubRuntimeState.Disconnected } as State,
+                statusReportEvent(statusToFlag(Status.UserProgramRunning)),
+            ).runtime,
+        ).toBe(HubRuntimeState.Disconnected);
+
+        // status update ignored while download not finished
+        expect(
+            reducers(
+                { runtime: HubRuntimeState.Loading } as State,
+                statusReportEvent(statusToFlag(Status.UserProgramRunning)),
+            ).runtime,
+        ).toBe(HubRuntimeState.Loading);
+
+        // normal operation - user program started
+        expect(
+            reducers(
+                { runtime: HubRuntimeState.Loaded } as State,
+                statusReportEvent(statusToFlag(Status.UserProgramRunning)),
+            ).runtime,
+        ).toBe(HubRuntimeState.Running);
+
+        // really short program run finished before receiving download finished
+        expect(
+            reducers({ runtime: HubRuntimeState.Loaded } as State, statusReportEvent(0))
+                .runtime,
+        ).toBe(HubRuntimeState.Idle);
+
+        // normal operation - user program stopped
+        expect(
+            reducers(
+                { runtime: HubRuntimeState.Running } as State,
+                statusReportEvent(0),
+            ).runtime,
+        ).toBe(HubRuntimeState.Idle);
+    });
+});
diff --git a/src/hub/reducers.ts b/src/hub/reducers.ts
index 822456c8..5f87654b 100644
--- a/src/hub/reducers.ts
+++ b/src/hub/reducers.ts
@@ -46,21 +46,38 @@ const runtime: Reducer<HubRuntimeState, Action> = (
         case BleDeviceActionType.DidDisconnect:
             return HubRuntimeState.Disconnected;
         case HubActionType.DidStartDownload:
+            // disconnected overrides download
+            if (state === HubRuntimeState.Disconnected) {
+                return state;
+            }
             return HubRuntimeState.Loading;
         case HubActionType.DidFinishDownload:
+            // disconnected overrides download
+            if (state === HubRuntimeState.Disconnected) {
+                return state;
+            }
             return HubRuntimeState.Loaded;
         case HubActionType.DidFailToFinishDownload:
+            // disconnected overrides download
+            if (state === HubRuntimeState.Disconnected) {
+                return state;
+            }
             return HubRuntimeState.Idle;
         case BlePybricksServiceEventActionType.StatusReport:
             // The loading state is determined solely by the IDE, so we can't
             // let the hub status interfere with it.
-            if (state !== HubRuntimeState.Loading) {
-                if (action.statusFlags & statusToFlag(Status.UserProgramRunning)) {
-                    return HubRuntimeState.Running;
-                }
-                return HubRuntimeState.Idle;
+            if (
+                state === HubRuntimeState.Disconnected ||
+                state === HubRuntimeState.Loading
+            ) {
+                return state;
             }
-            return state;
+
+            if (action.statusFlags & statusToFlag(Status.UserProgramRunning)) {
+                return HubRuntimeState.Running;
+            }
+
+            return HubRuntimeState.Idle;
         default:
             return state;
     }