Ah, yes... forgot to filter out library and blacklist games. Well I did a lot today so I'm allowed a bit of leeway ;)
I'll try to do that tonight... hopefully that won't hurt performance. Shouldn't. I mean, if barter does it quickly then why wouldn't I? That would be a first, ahah...

Small scrollbar: hmm... don't you have a mouse wheel..?
Can look into collapsing long lists too.

Not on my mobile, lol. But sweep with finger. I keep away long waiting times out of home by seeking trades :^^;:
Not that the size if the scroll matters hehehe. Long lists on a mobile are quite a problem.
Once you update, long long lists will not be a problem anymore.
Leeway-ing is good too ;)

The only thing I can remember is a custom tag for private notes, but it is not priority and I wouldn't know how much work is involved in that.

Already implemented at the same time as the store/quantity editor, but not enabled outside of me ;) mostly for policy reasons (I'll explain later.)

Can you add an option to auto-remove items from the Wishlist if they are already in Library? I am not after extra copies and it is very frustrating having to do the removal manually all the time. Especially because there is no button to remove all of them at once.

What do you mean, do you have that many to remove..?
It's just one click per game (you don't even have to visit game pages for that, you can do it all from your wishlist page...)

I agree with  @akyleen, the norm is not to wish for the ones we already have. They should be removed automatically, and added only as extra wishlist for the ones who want doubles.

The best I can do right now would be a 'Delete all' button...

That would help a lot :)

Not that it is important right now, but would a button allowing the Blacklist to synchronize with the Library be a possibility?

What do you mean?

@Akylen, I've spent my evening on this... It's not that it's hard to add a 'Delete all' button, it's that I wanted to do the entire operation in a single request, meaning I had to refactor my code to allow for multiple IDs to be passed (which I also need to later allow for mass tagging on list pages.)
Well, what a failure. It made everything more complicated-- my API went from ~30 lines of code up to 50. Thankfully it's just 50, eh..?
I also met a lot of bugs, I think I've tested for them all, but if you get an error while adding or removing something, or if the operation doesn't complete with the requested addition or removal, please be sure to PM me so I can look into it!
It's 1:20am here so I'm off to bed... Sorry I couldn't do more.