[folding] Add fade in/out animation for folding arrows?

[Tyriar]

The appearance/disappearance of the arrows is quite abrupt, a short fade similar to scroll bars would probably look nicer.

[aeschli]

@bgashler1 Assigning to you, Brad, to make a decision. If you like the suggestion, feel free to add, or assign back to me.

[bgashler1]

@aeschli This is a great idea 👍 I can push it tomorrow before end of day my time, if that's not too late. Let me know. Otherwise, we'll definitely get it by March.

[aeschli]

@bgashler1 Feel free to deliver!

[bgashler1]

Unfortunately, because we're using a parent hover technique, I don't believe it's possible to do a fade out. I think it's still valuable to do the fade in though—it makes things less abrupt. Hopefully we can figure out the fade out later, if it's possible.

[bgashler1]

Assigning to @aeschli to verify.

[Tyriar]

@bgashler1 I'm not understanding why fade out doesn't work. Seems like it would work fine since the parent isn't hiding or anything is it?

[joaomoreno]

Yeah, fade out is surely possible, pushed a change. Also sped up the opacity transition, it felt too slow.

[joaomoreno]

We reverted the commits since the opacity also fades out the git diff decorations. See #3383. Reverted commits are b0ca83f and 20d655f.

Reopening, but not for February.