webview.evaluateJavascript apparently just not running for long strings

Question

I have an android app I've programmed where most of the action happens in a html+javascript webView, and I have a couple of scenarios where I pass data and commands back and forth between the Java side and the WebView/JS side.

I've recently programmed a way for the JS side to trigger a registerForActivityResult(new ActivityResultContracts.GetContent(), ... call to allow the user to select an image, and it works, but when I try to send the data back from Java to my javascript, it fails.

public void onActivityResult(Uri uri) {
                    // this means I've got the URI selected from the gallery
                    try {
                        final InputStream imageStream = getContentResolver().openInputStream(uri);
                        final Bitmap selectedImage = BitmapFactory.decodeStream(imageStream);
                        ByteArrayOutputStream baos = new ByteArrayOutputStream();
                        selectedImage.compress(Bitmap.CompressFormat.JPEG, 100, baos);
                        byte[] b = baos.toByteArray();
                        String ecImage = Base64.encodeToString(b, Base64.DEFAULT);
                        
                        myWebView.evaluateJavascript("onImageSelectTest('" + ecImage + "')", null);
                        
                    } catch (FileNotFoundException e) {
                        // e.printStackTrace();
                    }
                }

After a bit of testing, I've found that it's only when the ecImage variable contains the base64 long string that the function doesn't run -- it runs just fine if I set ecImage = "abcd"; for example.

I want the webview to have the image for display purposes and, mainly, for posting the data to a web server, and right now for me the easiest way I can think of doing that is passing the image as a base64 string, but the evaluateJavascript call just isn't working with the long strings right now. I'm open to alternative ways of passing the image data to the webview.