My Applet computes its own window size. Can I resize it in the browser?

No, you cannot resize an applet's "view window" within an HTML page.

However, a simple computation can be written in JavaScript. If you have a complex computation, then it is in a separate class from your applet sub-class.

You can write a faceless applet (1x1 pixels) that uses the complex class to calculate the rectangle, and then use JavaScript to draw the pa/frame which loads the UI applet. Then, you can pass the computed data via a shared static data class used by both applets.