If you run the Swing example from the v1.3.0 release on the Windows platform, then resize the window, the Webview doesn't size to fill the frame and remains its original size. In the "direct" example (Example.java in v1.3.0), it does fill the frame when you resize the window.
I was going to see if this was still the case in the current main branch, but I was blocked by issue #29. However, I did steal the AWTWebview class from the current code and ran it with the v1.3.0 release. It seemed to show the same behavior.
Side note, great job on this project guys. It really opened up some possibilities for me in the Java world.