Corel basically copied Word’s design wholesale so I’m not surprised at this. Also as mentioned by @Jorpho design patents are different from utility patents. The functionality here isn’t novel of course - it’s specific design Microsoft uses for the contested UI elements that’s being disputed.
There’s nothing frivolous or unreasonable about this legal action in my opinion. What is outrageous is what happens if Corel loses the lawsuit (see Samsung vs Apple).
ETA: just look at the lawsuit filing. It’s not just the slider here, it’s about Corel basically completely copying Word’s entire design with a “Word mode”.
We all like to point the finger and say “slider patent lol” but I’m sure the BB staff wouldn’t like it if I created a (non-parody) blog about “stupendous things” featuring Jackhammer Jim as a mascot, used a pseudonym of “Dory Cocktrow” to post articles and had a “boingboing” mode that used an identical style.
Also lets not forget that Apple does the same thing to defend its design parents as well. Is it ok when they do this?
With that said, I think it’s insane that winning entitles you to all of the losers profits regardless of the severity of the violation. That’s like saying if I build a car and make an air vent that violates some patent that you should be entitled to all of my profits. That just seems unconscionable to me but that’s why I’m not a lawyer (or rich) I guess.