Checking viewport width and height incorrectly?

I’m running 1900x1080 on my laptop
Using my screen capture program, the viewport measures 1920x974 - all reasonable so far

But when I load a page and poll self.width and self.height in the Shown event, it tells me it’s 1280x649
And if I alter the page or (any control’s) .width to be anything larger than 1280, it does indeed go off screen to the right.

How do I get xojo to know about the actual resolution of my browser?