diff --git a/src/useCallViewKeyboardShortcuts.ts b/src/useCallViewKeyboardShortcuts.ts
index 9f4ba558..221e8a22 100644
--- a/src/useCallViewKeyboardShortcuts.ts
+++ b/src/useCallViewKeyboardShortcuts.ts
@@ -14,20 +14,17 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { RefObject, useCallback, useRef } from "react";
+import { RefObject, useCallback, useMemo, useRef } from "react";
 
 import { useEventTarget } from "./useEvents";
 
 /**
  * Determines whether focus is in the same part of the tree as the given
- * element (specifically, if an ancestor or descendant of it is focused).
+ * element (specifically, if the element or an ancestor of it is focused).
  */
 const mayReceiveKeyEvents = (e: HTMLElement): boolean => {
   const focusedElement = document.activeElement;
-  return (
-    focusedElement !== null &&
-    (focusedElement.contains(e) || e.contains(focusedElement))
-  );
+  return focusedElement !== null && focusedElement.contains(e);
 };
 
 export function useCallViewKeyboardShortcuts(
@@ -50,12 +47,17 @@ export function useCallViewKeyboardShortcuts(
         if (!mayReceiveKeyEvents(focusElement.current)) return;
 
         if (event.key === "m") {
+          event.preventDefault();
           toggleMicrophoneMuted();
         } else if (event.key == "v") {
+          event.preventDefault();
           toggleLocalVideoMuted();
-        } else if (event.key === " " && !spacebarHeld.current) {
-          spacebarHeld.current = true;
-          setMicrophoneMuted(false);
+        } else if (event.key === " ") {
+          event.preventDefault();
+          if (!spacebarHeld.current) {
+            spacebarHeld.current = true;
+            setMicrophoneMuted(false);
+          }
         }
       },
       [
@@ -65,6 +67,10 @@ export function useCallViewKeyboardShortcuts(
         setMicrophoneMuted,
       ],
     ),
+    // Because this is set on the window, to prevent shortcuts from activating
+    // another event callback at the same time, we need to preventDefault
+    // *before* child elements receive the event by using capture mode
+    useMemo(() => ({ capture: true }), []),
   );
 
   useEventTarget(
diff --git a/test/useCallViewKeyboardShortcuts-test.tsx b/test/useCallViewKeyboardShortcuts-test.tsx
new file mode 100644
index 00000000..c1826797
--- /dev/null
+++ b/test/useCallViewKeyboardShortcuts-test.tsx
@@ -0,0 +1,92 @@
+/*
+Copyright 2024 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { render } from "@testing-library/react";
+import { FC, useRef } from "react";
+import { test } from "vitest";
+import { Button } from "@vector-im/compound-web";
+import userEvent from "@testing-library/user-event";
+
+import { useCallViewKeyboardShortcuts } from "../src/useCallViewKeyboardShortcuts";
+
+interface TestComponentProps {
+  setMicrophoneMuted: (muted: boolean) => void;
+  onButtonClick?: () => void;
+}
+
+const TestComponent: FC<TestComponentProps> = ({
+  setMicrophoneMuted,
+  onButtonClick = (): void => {},
+}) => {
+  const ref = useRef<HTMLDivElement | null>(null);
+  useCallViewKeyboardShortcuts(
+    ref,
+    () => {},
+    () => {},
+    setMicrophoneMuted,
+  );
+  return (
+    <div ref={ref}>
+      <Button onClick={onButtonClick}>I'm a button</Button>
+    </div>
+  );
+};
+
+test("spacebar unmutes", async () => {
+  const user = userEvent.setup();
+  let muted = true;
+  render(<TestComponent setMicrophoneMuted={(m) => (muted = m)} />);
+
+  await user.keyboard("[Space>]");
+  expect(muted).toBe(false);
+  await user.keyboard("[/Space]");
+  expect(muted).toBe(true);
+});
+
+test("spacebar prioritizes pressing a button", async () => {
+  const user = userEvent.setup();
+  const setMuted = vi.fn();
+  const onClick = vi.fn();
+  render(
+    <TestComponent setMicrophoneMuted={setMuted} onButtonClick={onClick} />,
+  );
+
+  await user.tab(); // Focus the button
+  await user.keyboard("[Space]");
+  expect(setMuted).not.toBeCalled();
+  expect(onClick).toBeCalled();
+});
+
+test("unmuting happens in place of the default action", async () => {
+  const user = userEvent.setup();
+  const defaultPrevented = vi.fn();
+  // In the real application, we mostly just want the spacebar shortcut to avoid
+  // scrolling the page. But to test that here in JSDOM, we need some kind of
+  // container element that can be interactive and receive focus / keydown
+  // events. <video> is kind of a weird choice, but it'll do the job.
+  render(
+    <video
+      tabIndex={0}
+      onKeyDown={(e) => defaultPrevented(e.isDefaultPrevented())}
+    >
+      <TestComponent setMicrophoneMuted={() => {}} />
+    </video>,
+  );
+
+  await user.tab(); // Focus the <video>
+  await user.keyboard("[Space]");
+  expect(defaultPrevented).toBeCalledWith(true);
+});