"Fluff" time is for sitting in meetings or writing reports, as work functions, or for just looking busy. I intentionally avoid doing real work that requires any focus or creativity in the fluff slot. After six hours of real work, any more time spent doing the same thing starts to become counterproductive. So just leave it for tomorrow.
I have been more productive while explicitly dedicating 2 hours a day to getting nothing useful done, than when trying to stay focused on one thing for 8 hours, and ending up goofing off, getting sidetracked, or procrastinating for some amount of time anyway.
That's the theory, anyway. My current employer doesn't keep my pipelines reliably filled. During the dry spells, I have a hard time expanding the fluff slot to more than two hours, and end up doing stuff like coding stupid CLI games, or vectorizing the company logo, or setting up a new vanilla VM that I might not ever use.
Since I log my own hours I became full aware of this 6 hours "ceiling" in the long term, for a few days I could work for many more hours, but that would have a big price to pay in future productivity. In the beginning I felt bad, as if I was not able to produce as much as other people leading to a lot of anxiety. As I realized that my actual productivity was going up, compared to my older self and other peers, I embraced it.
Now I work 6 productive hours and to me that's 8 hours of work, my results are better month by month, my quality of life is better and my anxiety is more controlled. Everybody wins.