When setting the java log level to FINE or higher, the webview display is corrupted.
The testcase has been described in the JBS: https://bugs.openjdk.java.net/browse/JI-9006337

The problem could also be reproducible when running WebLauncher with the
-Djava.util.logging.config.file="logging.properties"
and the logging.properties file in the working directory:

handlers = java.util.logging.ConsoleHandler
.level=ALL
java.util.logging.ConsoleHandler.level = ALL

The problematic code was found in the rt\modules\web\src\main\java\com\sun\javafx\webkit\prism\WCGraphicsPrismContext.java:280 (WCGraphicsPrismContext.setClip())
where the clipping bounds are attempted to be drawn (my guess) if the log level is set to FINE or higher.
Commenting this code resolves the problem.