Huh. I just checked. On Google Chrome, the preferred browser of this correspondent, I have magnification set at 125%.
Putting it back to 100% makes the differences in fonts much less noticeable (possibly because so tiny), and the black bar still creeps up (21" monitor, 1680x1050 resolution, but I don't have the browser full screen...)
No such issues on my phone, btw.
Ergo, perhaps not the forum settings but my own.