I'm not sure what's causing this... I've seen it in both dual and single file display modes after initially thinking it was specific to dual-display mode...
But there are times when for some reason the new 'Reset Zoom' control that pops up in the path field (which I think is great by the way, thanks for it) when you change the font scale just doesn't appear.
I can't tie it to anything specific that I'm doing... so if anyone else notices, please try and figure out what's causing it 
. My laptop mouse pad sensitivity goes a bit wonky frequently and I am OFTEN changing the font size unintentionally - so I quite like the new reset zoom control popping up when it needs to.