Fix mobile browser tab tombstoning issue in sandbox renderer

[Copilot]

Fixes an issue where on mobile browsers, the host package sandbox renderer does not show content if the tab was tombstoned and revisited.

Problem

Mobile browsers can "tombstone" tabs to save memory by pausing JavaScript execution and cleaning up resources. When this happens:

• Blob URLs created with URL.createObjectURL() may be garbage collected
• The iframe content using these blob URLs becomes inaccessible
• Users lose their rendered Chartifact documents when returning to the tab

Solution

This PR adds page visibility API event handling to automatically detect when a tab is restored from tombstoning and recreate the sandbox content if needed.

Key Changes

1. Content Storage: Store the current markdown content in currentMarkdown property for potential restoration
2. Visibility Events: Listen for visibilitychange events to detect when tab becomes visible again
3. Functionality Check: Added isSandboxFunctional() method to test if the sandbox iframe is still accessible
4. Auto-Restoration: Recreate the sandbox with stored content if the iframe is no longer functional
5. Cleanup: Added proper event listener cleanup in the destroy() method

Implementation Details

The fix is implemented entirely in packages/host/src/listener.ts with minimal, surgical changes:

// Store content for restoration
private currentMarkdown: string = '';
private visibilityChangeHandler: () => void;

// Setup page visibility handling
private setupPageVisibilityHandling() {
  this.visibilityChangeHandler = () => {
    if (document.visibilityState === 'visible') {
      this.handlePageBecameVisible();
    }
  };
  document.addEventListener('visibilitychange', this.visibilityChangeHandler);
}

// Check and restore if needed
private handlePageBecameVisible() {
  if (this.currentMarkdown && this.sandbox) {
    if (!this.isSandboxFunctional()) {
      this.createSandbox(this.currentMarkdown);
      show(this.sandbox.element, true);
    }
  }
}

Testing

Created and validated a comprehensive test that demonstrates:

• Content loads successfully
• Visibility change simulation works correctly
• Tombstoning detection and restoration logic functions properly
• Content persists after simulated tombstoning
• Sandbox remains functional after restoration

Impact

• Zero breaking changes - Only adds functionality without modifying existing behavior
• Backward compatible - Works with existing code without any changes needed
• Automatic activation - The fix activates automatically when needed, no user intervention required
• Mobile-friendly - Specifically addresses mobile browser limitations

The fix ensures that Chartifact interactive documents remain accessible and functional even after mobile browser tab tombstoning, providing a much better user experience on mobile devices.

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[Dan Marshall (danmarshall)]

Copilot although this was merged and branch deleted, but problem still isn’t solved