When "Fade unit frames" (under Unit Frames options) is enabled, your unit frame, target etc. disappear until you interact with something again.
But classes that have pets also have a pet bar, and it doesn't fade along with the rest of the frames:
see picture. This leaves a pet bar in the middle of the screen with nothing else.
There is the option to have the horizontal pet bar on mouseover, but
it is not the same as fading: you have to put your cursor on top of the pet bar to make it appear again.
When "Fade unit frames" (under Unit Frames options) & "Enable horizontal pet bar" (under Action Bars options) are enabled at the same time, the fade effect should apply to the pet bar as well. That way, when you interact with a target, your pet bar reappears at the same time as the unit frames etc.
Does this make sense?
Thank you!