If I put a web rectangle on a default web project using Xojo2021r31, and set the lock left and right, and then also place a button on the control, when I run the project, and click on the button I have put logic in the pressed event to retrieve the width of the rectangle. It reports the design time width. Not the actual width of whatever the page might be.
Then if I grab the right edge of the browser window and change it slightly so the window resize event is called and press the button again, now it reports the proper size.