diff --git a/app/javascript/utilities/__tests__/codeFullscreenModeSwitcher.test.js b/app/javascript/utilities/__tests__/codeFullscreenModeSwitcher.test.js
new file mode 100644
index 000000000..70f50da44
--- /dev/null
+++ b/app/javascript/utilities/__tests__/codeFullscreenModeSwitcher.test.js
@@ -0,0 +1,106 @@
+const importModule = () => import('@utilities/codeFullscreenModeSwitcher');
+
+describe('CodeFullScreenModeSwitcher Utility', () => {
+  let addFullScreenModeControl, getFullScreenModeStatus;
+  let onPressEscape, onPopstate;
+
+  const getFullScreenElements = () =>
+    ['.js-fullscreen-code.is-open', '.js-code-highlight.is-fullscreen'].map(
+      (selector) => document.body.querySelector(selector),
+    );
+
+  const testFullScreenElements = ({ exists }) => {
+    for (const element of getFullScreenElements()) {
+      if (exists) expect(element).not.toBeNull();
+      else expect(element).toBeNull();
+    }
+  };
+
+  const testNonFullScreen = () => {
+    expect(getFullScreenModeStatus()).toBe(false);
+    testFullScreenElements({ exists: false });
+    expect(document.body.style.overflow).toBe('');
+  };
+
+  const testFullScreen = () => {
+    expect(getFullScreenModeStatus()).toBe(true);
+    testFullScreenElements({ exists: true });
+    expect(document.body.style.overflow).toBe('hidden');
+  };
+
+  const getEnterFullScreenButtons = () =>
+    document.getElementsByClassName('js-fullscreen-code-action');
+
+  beforeAll(() => {
+    global.scrollTo = jest.fn();
+
+    document.body.innerHTML = `
+      <div class="js-code-highlight">
+        <div class="js-fullscreen-code-action"><div>
+      </div>
+      <div class="js-fullscreen-code"></div>
+    `;
+  });
+
+  beforeAll(async () => {
+    ({ addFullScreenModeControl, getFullScreenModeStatus } =
+      await importModule());
+    addFullScreenModeControl(getEnterFullScreenButtons());
+  });
+
+  beforeAll(async () => {
+    // eventListener functions
+    ({ onPressEscape, onPopstate } = await importModule());
+  });
+
+  it('starts in non-fullscreen mode', testNonFullScreen);
+
+  it('enters fullscreen mode on click', () => {
+    const goFullScreenButtons = getEnterFullScreenButtons();
+    const spyDoc = jest.spyOn(document.body, 'addEventListener');
+    const spyWindow = jest.spyOn(window, 'addEventListener');
+
+    goFullScreenButtons[0].click();
+
+    testFullScreen();
+    expect(spyDoc).toHaveBeenCalledWith('keyup', onPressEscape);
+    expect(spyWindow).toHaveBeenCalledWith('popstate', onPopstate);
+  });
+
+  it('exits fullscreen mode on Escape key', () => {
+    const spyDocRemove = jest.spyOn(document.body, 'removeEventListener');
+    const spyWindowRemove = jest.spyOn(window, 'removeEventListener');
+
+    testFullScreen();
+    document.body.dispatchEvent(new KeyboardEvent('keyup', { key: 'Escape' }));
+    testNonFullScreen();
+
+    expect(spyDocRemove).toHaveBeenCalledWith('keyup', onPressEscape);
+    expect(spyWindowRemove).toHaveBeenCalledWith('popstate', onPopstate);
+  });
+
+  it('re-enters fullscreen mode on click', () => {
+    const goFullScreenButtons = getEnterFullScreenButtons();
+    const spyDoc = jest.spyOn(document.body, 'addEventListener');
+    const spyWindow = jest.spyOn(window, 'addEventListener');
+
+    testNonFullScreen();
+    goFullScreenButtons[0].click();
+    testFullScreen();
+
+    expect(spyDoc).toHaveBeenCalledWith('keyup', onPressEscape);
+    expect(spyWindow).toHaveBeenCalledWith('popstate', onPopstate);
+  });
+
+  it('exits fullscreen mode on popstate event', () => {
+    const spyDocRemove = jest.spyOn(document.body, 'removeEventListener');
+    const spyWindowRemove = jest.spyOn(window, 'removeEventListener');
+
+    testFullScreen();
+    window.dispatchEvent(new PopStateEvent('popstate', { state: { key: '' } }));
+    testNonFullScreen();
+
+    expect(spyDocRemove).toHaveBeenCalledWith('keyup', onPressEscape);
+    expect(spyWindowRemove).toHaveBeenCalledWith('popstate', onPopstate);
+  });
+});
diff --git a/app/javascript/utilities/codeFullscreenModeSwitcher.js b/app/javascript/utilities/codeFullscreenModeSwitcher.js
index 14d6a3526..5e42a6404 100644
--- a/app/javascript/utilities/codeFullscreenModeSwitcher.js
+++ b/app/javascript/utilities/codeFullscreenModeSwitcher.js
@@ -4,6 +4,10 @@ const fullScreenWindow =
   document.getElementsByClassName('js-fullscreen-code')[0];
 const { body } = document;
 
+export function getFullScreenModeStatus() {
+  return isFullScreenModeCodeOn;
+}
+
 function setAfterFullScreenScrollPosition() {
   window.scrollTo(0, screenScroll);
 }
@@ -12,7 +16,7 @@ function getBeforeFullScreenScrollPosition() {
   screenScroll = window.scrollY;
 }
 
-function onPressEscape(event) {
+export function onPressEscape(event) {
   if (event.key == 'Escape') {
     fullScreenModeControl(event);
   }
@@ -26,6 +30,18 @@ function listenToKeyboardForEscape(listen) {
   }
 }
 
+export function onPopstate() {
+  fullScreenModeControl();
+}
+
+function listenToWindowForPopstate(listen) {
+  if (listen) {
+    window.addEventListener('popstate', onPopstate);
+  } else {
+    window.removeEventListener('popstate', onPopstate);
+  }
+}
+
 function toggleOverflowForDocument(overflow) {
   if (overflow) {
     body.style.overflow = 'hidden';
@@ -51,7 +67,7 @@ function removeFullScreenModeControl(elements) {
 }
 
 function fullScreenModeControl(event) {
-  const codeBlock = event.currentTarget.closest('.js-code-highlight')
+  const codeBlock = event?.currentTarget.closest('.js-code-highlight')
     ? event.currentTarget.closest('.js-code-highlight').cloneNode(true)
     : null;
   const codeBlockControls = codeBlock
@@ -62,6 +78,7 @@ function fullScreenModeControl(event) {
     toggleOverflowForDocument(false);
     setAfterFullScreenScrollPosition();
     listenToKeyboardForEscape(false);
+    listenToWindowForPopstate(false);
     removeFullScreenModeControl(codeBlockControls);
 
     fullScreenWindow.classList.remove('is-open');
@@ -72,6 +89,8 @@ function fullScreenModeControl(event) {
     toggleOverflowForDocument(true);
     getBeforeFullScreenScrollPosition();
     listenToKeyboardForEscape(true);
+    listenToWindowForPopstate(true);
+
     codeBlock.classList.add('is-fullscreen');
     fullScreenWindow.appendChild(codeBlock);
     fullScreenWindow.classList.add('is-open');