Fullscreen button of Video inside WebView not working

I'm trying to implement a subclass of WebChromeClient in order to have a working fullscreen button on youtube embed video links (ex: https://www.youtube.com/embed/dQw4w9WgXcQ) inside a WebView.

I basically simplified this repo, here are some snippets:

VideoWebChromeClient:

public class VideoWebChromeClient extends WebChromeClient {
    private boolean isVideoFullscreen = false;
    private View activityNonVideoView;
    private ViewGroup activityVideoView;
    private View videoViewContainer;
    private CustomViewCallback videoCallback;
    private Window videoWindow;

    public VideoWebChromeClient(View activityNonVideoView, ViewGroup activityVideoView, Window window) {
        this.activityNonVideoView = activityNonVideoView;
        this.activityVideoView =  activityVideoView;
        this.videoWindow = window;
    }

    @Override
    public void onShowCustomView(View view, CustomViewCallback callback) {
        Log.w("ENTER FULLSCREEN");
        videoCallback = callback;
        videoViewContainer = view;
        videoWindow.addFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN | WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
        activityNonVideoView.setVisibility(View.GONE);
        activityVideoView.addView(videoViewContainer, new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
        activityVideoView.setVisibility(View.VISIBLE);
        isVideoFullscreen = true;
    }

    @Override
    public void onHideCustomView() {
        if (!isVideoFullscreen) {
            return;
        }
        Log.w("EXIT FULLSCREEN");
        activityVideoView.setVisibility(View.GONE);
        activityVideoView.removeView(videoViewContainer);
        activityNonVideoView.setVisibility(View.VISIBLE);
        videoWindow.clearFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN | WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
        videoViewContainer = null;
        videoCallback.onCustomViewHidden();
        isVideoFullscreen = false;
    }

    public boolean onBackPressed() {
        onHideCustomView();
        return isVideoFullscreen;
    }
  }

WebViewActivity

public class WebViewActivity extends BaseActivity {

    public static final String WEB_VIEW_URL_EXTRA = "URL";
    private boolean loadedFirstURL = true;
    private VideoWebChromeClient mWebChromeClient;

    @Bind(webview)
    WebView mWebView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(provideLayoutId());
        String url = (String) getIntent().getSerializableExtra(WEB_VIEW_URL_EXTRA);

        // Check if URL is YouTube / Vimeo
        if (RegexHelper.isVideoURL(url)){
            // Allow Fullscreen
            final View defaultLayout = findViewById(R.id.defaultLayout);
            final View fullscreenLayout = findViewById(R.id.fullscreenLayout);
            mWebChromeClient = new VideoWebChromeClient(defaultLayout, (ViewGroup) fullscreenLayout, getWindow());
            mWebView.setWebChromeClient(mWebChromeClient);

            // Block external links
            mWebView.setWebViewClient(new WebViewClient() {
                @Override
                public boolean shouldOverrideUrlLoading(WebView view, String url) {
                    if (loadedFirstURL) {
                        loadedFirstURL = false;
                        return false;
                    }
                    return true;
                }
            });
        }
        mWebView.getSettings().setJavaScriptEnabled(true);
        mWebView.loadUrl(url);
    }

        @Override
        public void onBackPressed() {
            if (!mWebChromeClient.onBackPressed())
            {
                if (mWebView.canGoBack()) {
                    mWebView.goBack();
                }
                else {
                    super.onBackPressed();
                }
            }
        }
    }

It works correctly on KitKat but not on Nougat (I have only those two devices to test on): when I tap the fullscreen button it sometimes goes into fullscreen, but most times it "flashes" and stays not-fullscreen.

And then, when it's actually in fullscreen mode, the button to exit fullscreen doesn't work (onHideCustomView is not called at all)

You can see that I put logs and noticed that when the "flash" happens, onHideCustomView is actually called right after onShowCustomView ("ENTER FULLSCREEN" then immediately "EXIT FULLSCREEN"), I don't understand why.

Thanks a lot if anyone can point me towards a solution