Flutter WebView (flutter_inappwebview 6.1.0) not loading external JS script in React page — blank screen issue for few users only

Question

I have a Flutter app using flutter_inappwebview: ^6.1.0. The frontend is built in Flutter, and the backend is Spring Boot (Java).

In the Flutter app, I load a WebView using initialUrlRequest, pointing to a React-based webpage (form.tsx). The React page tries to dynamically load a JavaScript file via an API endpoint, then notifies Flutter when it's rendered.

React code(form.tsx)

const script: HTMLScriptElement = document.createElement('script');
script.src = formScriptUrl; // loaded via axios blob: "/down?formPath=/resources/example/form-ex.js"
script.defer = true;
script.onload = function () {
  window.flutter_inappwebview.callHandler('formRendered');
};
document.body.appendChild(script);

The JS file is fetched from a Spring API:

return await axios( {
  url: '/down?formPath=/resources/example/form-ex.js',
  method: 'GET',
  responseType: 'blob',
  headers: {
    'Content-Type': 'text/javascript'
    'Accept': 'application/json',
  },
});

Once the formRendered signal is received in Flutter, we run this:

await _webViewController?.evaluateJavascript(source: 'getBodyHtml()');

Where getBodyHtml() is:

const formEl = document.getElementById("body-html-area");
const BodyHtml = formEl?.innerHTML || '';
return BodyHtml;

Problem:

No API call is made to /down?formPath=.... The server has no request or error logs.
We wrapped the script logic in try-catch, with an alert() in case of errors — but no alert shows.
The WebView just shows a blank screen.
This issue occurs only for 2~3 users. Most users have no problems.

Devices include both Android and iOS.
Minimum Android version observed: 14
No consistent pattern (e.g., Galaxy S23 works for some, fails for others)

What I've tried:

Confirmed the JS file loads fine when accessed directly in browsers.
Confirmed window.flutter_inappwebview.callHandler() works for most users.
Verified getBodyHtml() works in normal conditions.
Confirmed correct MIME type (text/javascript) and response format (blob).
Added alert() statements in the script load path — but nothing is triggered for affected users.
No CORS error is shown (and most users work fine, so it shouldn't be a policy issue).

What could cause this script loading to silently fail on only a few devices?

Is there something about how blobs, script injection, or WebView caching works differently on a few devices or under rare network conditions?

Tejas Soni · Accepted Answer · 2025-06-19 13:08:21Z

0

Some WebView implementations (especially on newer Android/iOS versions) restrict loading scripts from blob URLs for security reasons.

If you fetch the JS as a blob and try to inject it as a <script src="blob:...">, it may silently fail on some devices.

Instead of using a blob URL, fetch the JS as text and inject it directly:

const response = await axios.get('/down?formPath=/resources/example/form-ex.js', { responseType: 'text' });

const script = document.createElement('script');

script.text = response.data;

document.body.appendChild(script);

Try disabling cache in your WebView:

InAppWebView(
  initialOptions: InAppWebViewGroupOptions(
    crossPlatform: InAppWebViewOptions(
      cacheEnabled: false,
      clearCache: true,
    ),
  ),
  // ...
)

Also, clear Service Worker caches in your React app if used.

Check your CSP and allow 'unsafe-inline' or 'unsafe-eval' for testing.

answered Jun 19 at 13:08

Tejas Soni

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1 Comment

Lee Jun 20 at 3:39

But if it was received as a blob, that means the API call to /down?formPath=... was made. However, there are no request logs or error logs on the server.

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