Thanks for ruling out Javascript as the culprit, this saved me a lot of time, and I appreciate the prompt reply.
Indeed, in modifying the zenpage theme, I must have introduced some sort of anomaly which caused the expansion of the browser window to almost twice the proper size.
I did set the html tag to a max-width of 900px in the header section of pages.php, as well as in every reference to <html> or <body> in style.css. Since all my other property assignments to the <html> tag appeared to be working properly, I remain mystified as to how that property could have been overridden. I also noticed that on a test platform where I have an instance of IE version 6, the browser window did not expand. (In all later versions of IE, as in Firefox, I saw the same problem.)
Having now upgraded to Zen-1.4.4.3, I plan to retrace my steps modifying the Zenpage theme, and see if I can figure out what caused this.
Thanks again for your assistance.