Reduce layout quirks on mobile devices.

Something's still missing, I suspect it has to do with device-width,
but this causes fewer issues by removing the fixed elements when the
viewport becomes too narrow. I'm still noticing issues on tablets, but
at least the content is always readable at default zoom on tablets, so
unintentional breakage is less likely until I find a proper fix.
(Or I could simply move away from a fixed layout when device-width < 960px.)