I increased the text size of this post because it started uncomfortably small (for me). Curiosity made me keep increasing the text size until it hit both ends of my browser, then I increased it some more and the text disappeared off the ends. The text did reflow as the size increased, within some internal margin set by the site's style, so it's clearly possible to reflow within the bounds of the window.
Why doesn't it reflow to stay inside the window, and why is this considered good? I vaguely recall that it used to be normal to reflow and keep everything visible.
I checked with FF and Chromium on linux, same behavior.