I normally have my laptop set up with a second screen plugged in, so my desktop is 1280x768 (laptop screen) and 1024x768 (second screen)
I know many people have smaller screens but even the 1024 pixel wide screen shows a horizontal bar because the MN page appears to require the full width (ie not allowing space for the vertical scroll bar).
Is this just a quirk of Google Chrome or by design ? Not sure if all users can have the option of even 1024 pixels wide (must check my G3 iMac, which admittedly is dated, but one of the intentions of web page design was not to 'restrict' users to needing particular browsers or screen resolution... maybe Mumsnet just decided to ignore that option).