Font has changed

[iAlexG]

on my mac I have noticed on the menu bar the drop down menus font has changed when you hover of an option. But it is only on the Finder or right clicking on the desktop or right clicking on an app icon. When I hover over an option the font changed from bold to regular

 

[angelwatt]

And? It's actually a perceptual illusion, the font's boldness isn't different.