fix(core): add fallback handler for hardware back button when no router is present (#30878)

Issue number: internal

---------

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## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

This pull request enhances the hardware back button functionality to
ensure a consistent user experience, especially when no custom handlers
are registered. The main improvement is the addition of a fallback
handler that triggers the default browser back navigation when no other
handlers are present.

**Hardware Back Button Improvements:**

* Added a fallback handler in `startHardwareBackButton` that navigates
back in browser history (`win?.history.back()`) if no custom handlers
are registered, ensuring the hardware back button always performs a
meaningful action.
* Introduced a constant `FALLBACK_BACK_BUTTON_PRIORITY` with a value of
`-1` to manage the priority of the fallback handler.

**Code Consistency:**

* Moved the import of `win` from `@utils/browser` to group it with other
imports for consistency.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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  2. Update the BREAKING.md file with the breaking change.
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## Other information

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screenshots of how the component looks before and after the change. -->

---------

Co-authored-by: ShaneK <shane@shanessite.net>

core/src/utils/hardware-back-button.ts

@@ -1,5 +1,5 @@
-import { win } from '@utils/browser';
 import type { CloseWatcher } from '@utils/browser';
+import { win } from '@utils/browser';
 import { printIonError } from '@utils/logging';
 
 import { config } from '../global/config';
@@ -69,6 +69,21 @@ export const startHardwareBackButton = () => {
     });
     doc.dispatchEvent(ev);
 
+    /**
+     * If no handlers have been registered, fall back to the default
+     * behavior of navigating back in history. This ensures the hardware
+     * back button works even when no router or custom handler is present.
+     */
+    if (handlers.length === 0) {
+      handlers.push({
+        priority: FALLBACK_BACK_BUTTON_PRIORITY,
+        handler: () => {
+          win?.history.back();
+        },
+        id: index++,
+      });
+    }
+
     const executeAction = async (handlerRegister: HandlerRegister | undefined) => {
       try {
         if (handlerRegister?.handler) {
@@ -138,3 +153,4 @@ export const startHardwareBackButton = () => {
 
 export const OVERLAY_BACK_BUTTON_PRIORITY = 100;
 export const MENU_BACK_BUTTON_PRIORITY = 99; // 1 less than overlay priority since menu is displayed behind overlays
+const FALLBACK_BACK_BUTTON_PRIORITY = -1; // Fallback when no other handlers are registered

core/src/utils/test/hardware-back-button.spec.ts

@@ -54,6 +54,47 @@ describe('Hardware Back Button', () => {
     dispatchBackButtonEvent();
     expect(cbSpyTwo).toHaveBeenCalled();
   });
+
+  it('should fall back to history.back() when no handlers are registered', () => {
+    const historyBackSpy = jest.fn();
+    const originalBack = win?.history?.back;
+    if (win?.history) {
+      win.history.back = historyBackSpy;
+    }
+
+    // Don't register any ionBackButton handlers
+    dispatchBackButtonEvent();
+
+    expect(historyBackSpy).toHaveBeenCalled();
+
+    // Restore original
+    if (win?.history && originalBack) {
+      win.history.back = originalBack;
+    }
+  });
+
+  it('should not call history.back() when a handler is registered', () => {
+    const historyBackSpy = jest.fn();
+    const originalBack = win?.history?.back;
+    if (win?.history) {
+      win.history.back = historyBackSpy;
+    }
+
+    const cbSpy = jest.fn();
+    document.addEventListener('ionBackButton', (ev) => {
+      (ev as BackButtonEvent).detail.register(0, cbSpy);
+    });
+
+    dispatchBackButtonEvent();
+
+    expect(cbSpy).toHaveBeenCalled();
+    expect(historyBackSpy).not.toHaveBeenCalled();
+
+    // Restore original
+    if (win?.history && originalBack) {
+      win.history.back = originalBack;
+    }
+  });
 });
 
 describe('Experimental Close Watcher', () => {