Summary

This PR enables support for file:// URIs with relative paths in iOS and macOS WebView implementations by configuring WKWebView to allow file access from file URLs. This fixes the issue where local HTML files could not load additional resources (CSS, JavaScript, images) using relative paths.

Problem

By default, WKWebView blocks file URLs from accessing other file URLs for security reasons. This prevents legitimate use cases such as:

• Loading a PDF.js viewer with a local PDF file as a parameter
• Bundled web applications with CSS/JS/image assets referenced via relative paths
• HTML documentation viewers with relative resource links
• Any offline web content that needs to reference multiple local files

Solution

Following the approach from .NET MAUI #19486, this PR enables the allowFileAccessFromFileURLs preference on WKWebView configuration during initialization.

Changes Made

1. iOS/macOS Native WebView (UnoWKWebView.iOSmacOS.cs)

   Configuration.Preferences.SetValueForKey(NSObject.FromObject(true), (NSString)"allowFileAccessFromFileURLs");

2. macOS Skia Native WebView (UNOWebView.m)

   [config.preferences setValue:@YES forKey:@"allowFileAccessFromFileURLs"];

3. Runtime Test (Given_WebView.cs)

   • Added When_FileUri_With_Relative_Paths test that verifies file:// URIs can load relative resources
   • Test creates an HTML file that references a CSS file via relative path
   • Validates both the HTML and CSS load successfully, confirming allowFileAccessFromFileURLs works correctly

All changes are applied during WebView initialization, ensuring consistent behavior across iOS and macOS platforms.

Use Cases

This enables scenarios like:

// Load PDF.js viewer with a local PDF
webView.Source = new Uri($"file://{pdfViewerPath}?file=file://{pdfFilePath}");

// Load local HTML with relative CSS/JS
webView.Source = new Uri($"file://{htmlPath}");
// where the HTML contains: <link rel="stylesheet" href="./styles.css">

Testing

• ✅ Runtime test validates file:// URIs with relative paths work on iOS and macOS
• ✅ Test confirms CSS files are loaded via relative links
• ✅ Test verifies the configuration change achieves the intended functionality

Security Considerations

This change follows the same security model as .NET MAUI and only affects file:// URLs. HTTP/HTTPS content security remains unchanged.

References

• Related MAUI issue: .NET MAUI project using the PDF.js library - Issue loading the PDF from iOS resource dotnet/maui#19486 (comment)
• Additional context: Webview on iOS and Android does not allow local HTML to load additional resources dotnet/maui#20613 (comment)

Fixes #21572

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