[Bug]: MetaMask defaults to side panel in Arc, preventing the wallet popup from opening

Nobody has claimed this yet.

### Describe the bug

In MetaMask Extension `13.28.0` on Arc Browser for macOS, MetaMask now defaults the toolbar action to Chrome's side panel. Arc appears to expose enough Chromium side-panel state for MetaMask to enable this path, but Arc's extension side-panel support is broken for this use case. The result is that clicking the MetaMask toolbar icon does not open MetaMask at all.

Switching MetaMask's own persisted preference back to popup mode fixes the issue:

```text
PreferencesController.preferences.useSidePanelAsDefault = false
```

I published a small open-source workaround that does only that, with dry-run mode and LevelDB backups:

https://github.com/boyuanx/metamask-arc-popup-fix

The workaround is not meant to be the long-term fix; it is a reproducible demonstration that popup mode still works and that the side-panel default is what breaks Arc users.

### Expected behavior

MetaMask should continue opening as a popup in Arc, or should fall back to popup mode when side-panel opening is unavailable or broken.

More generally, MetaMask should avoid making side panel the default solely because `chrome.sidePanel` exists. Some Chromium-based browsers may expose the API or related persisted state without providing a working extension side-panel UX.

### Screenshots/Recordings

N/A.

### Steps to reproduce

1. Use Arc Browser on macOS.
2. Install or update MetaMask Extension to `13.28.0` from the official extension store.
3. Click the MetaMask toolbar icon.
4. Observe that MetaMask does not open.
5. Change MetaMask's persisted preference `PreferencesController.preferences.useSidePanelAsDefault` to `false`.
6. Reopen Arc and click the MetaMask toolbar icon again.
7. Observe that MetaMask opens correctly as a popup.

### Error messages or log output

No visible UI error is shown to the user. The toolbar click simply does not open MetaMask.

### Where was this bug found?

Live version (from official store)

### Version

13.28.0

### Build type

Official store build

### Browser

Other: Arc Browser, Chromium-based

### Operating system

MacOS

### Hardware wallet

N/A

### Additional context

From inspecting the installed extension behavior, MetaMask appears to set the side-panel toolbar behavior via `chrome.sidePanel.setPanelBehavior({ openPanelOnActionClick: ... })`, with the default ultimately tied to `preferences.useSidePanelAsDefault`.

For Arc, setting that preference to `false` restores the expected popup behavior. The helper repo linked above automates that preference change without patching the signed MetaMask extension bundle.

### Severity

This blocks normal wallet access from the browser toolbar for affected Arc users. Users can reasonably interpret this as MetaMask being broken or inaccessible after the update.

Start by reproducing the toolbar-click failure in Arc Browser on macOS with MetaMask 13.28.0, then inspect the side-panel behavior and PreferencesController.preferences.useSidePanelAsDefault described in the issue. Done means the toolbar opens MetaMask as a popup in Arc, or reliably falls back to popup mode when side-panel opening is unavailable.

Written by the indexing model from the issue text.