Perhaps you have to reboot after un-tinkering. There is also no harm in putting the minimum font size back to where it was. That happened on the fly for me which impressed me, live on what ever page I had open.

Good Luck

I logged out, but didn't reboot.

OK, I'll have an, er, tinker then..

Is it possible to run BOTH the new Dell and the old Samsung S23B550 off this mac Mini, whilst preserving the 2560 x 1440 output to the Dell monitor?

I plugged in the Sammy via HDMI and both monitors are set to "best for display" resolution.

This may come in handy. Why Apple took away options i know not.
Check you have extended desktop not mirror setup.

https://itunes.apple.com/gb/app/disp...49083868?mt=12

Thanks!

I'm not mirroring, just have extended with the Menu Bar visible at the top of each screen.

For some reason the Menu Bar is very pale and translucent in the old Sammy display - is that just because I have become used to the Dell now?

On a two-monitor setup in Mavericks, only one menu bar will be "active" at any one time. The other one will appear very faint. You can still click on it, but it's not the "current" one.
Whichever monitor has the front-most window with focus, has the active menu bar.