Behaviour of sub/sup features

[Reinmar]

The https://github.com/ckeditor/ckeditor5-basic-styles/pull/74 PR by @idleb introduces basic subscript and superscript support. However, since it's implemented in the same way than bold/italic/etc it comes with their semantics:

1. The order in which you apply styles does not count – they are always output in the same order (i > strong). With sub/sup you may want to create sup > sub or sub > sup. It simply depends on the semantics of the text and cannot be hardcoded by the editor.
2. You cannot apply sup or sub twice to the same text. It is possible (although very unlikely) that you may want to write sub > sub or sup > sub > sup. That won't be doable with how this feature is implemented.

In CKEditor 4 the (2) is not possible but (1), the order, is controllable. In CKEditor 5 both are not doable now and while we can ignore (2) I'm afraid about (1). The biggest problem here is that it won't be that small change. Also, regarding (2), I remember that it was requested by the users of CKEditor 4 (IDK how many though).

But, before we'll sink into this, are these limitations important to you? Please 👍 this ticket and leave comments which parts are crucial for you.

[Reinmar]

Regarding the order (1) – the current order how e.g. bold and italic are output base, I think, on the alphabetical order (so to be consistent). There's also the concept of priorities of AttributeElements (and it's e.g. used by the link feature to output <a> as the outermost element), but in this case, both elements have the same priority so I think we use alphabetical sort.

It came to my mind though, that we could use current date as a value of sub/sup model attributes and use it later, with conjuction with some base priority to change the order of sub and sup in the view. For example, let's say the base priority is 10. So, sub created at new Date().getTime() has priority:

10 + new Date().getTime() / 10000000000000 ~= 10.1535966907083

A couple of seconds later:

10.1535966958953

Etc. With every second it gets a higher priority so sub/sups created later get inside (or outside, however it should work) of sub/sups created earlier.

I wonder if this may work :D It seems to be a hack but perhaps once we analyse this it may turn out that it's indeed what we need.

PS. The only problem is if you have two different sub values next to each other. Do we want to merge them? If yes, then the above algorithm needs to be reviewed.

[scofalik]

It won't work for removing (unwrapping) such AttributeElement without some additional work. Because of different priorities.

[Reinmar]

Why? The priority will be the same – it'll be taken from the sub/sup model attr. I want to store the date in the model.

[scofalik]

Oh sorry, missed that part.

[scofalik]

As far as creating sub in sub and sup in sup: how do you see UX working for that? When should the button be "clicked" so you are able to remove sub/sup instead of creating a new one? When exactly the whole sub/sup element is selected?

[pjasiun]

I think that to implement it, we need to add some priority information, most probably also to the model, and then take it into consideration during the conversion.

I think, instead of using the timestamp we could check what priorities other elements has and use an incremented value.

[Reinmar]

Answering @scofalik. Check out CKEditor 4. When you're in:

<p><sub>Foo[Bar]</sub></p>

And you:

• click the Sub button you remove that style,
• click the Sup button you get: <p><sub>Foo<sup>[Bar]</sup></sub></p>

So, the order of styles is that the style which is applied later ends inside the style applied earlier.

[Reinmar]

Answering @pjasiun.

I think, instead of using the timestamp we could check what priorities other elements has and use an incremented value.

How's that different from using a timestamp? You know the base priority of Sub/Sup. So when you want to decrement/increment, you just need to add whatever to it.

[pjasiun]

The difference is not big. I just think it is nicer to use 11 than 10.1535966958953. And it should give us smaller messages in collaboration, but that basically it.

[Reinmar]

Hard to tell whether int priorities are going to work in a longer term. Depending on how we'd use them when applying and removing the styles a couple of time we might quickly end up with priorites like 20 and that would affect how this feature is rendered next to some other inline styling. That's why I'd try to operate within a small range like 10-11. But we do not have to use timestamps for that naturally. I just thought it may a trick worth considering.

[quicksketch]

I think <sub> and <sup> tags should be made mutually-exclusive. If you click the Subscript button on Superscripted text, it should switch from one tag to the other. While it makes a lot of sense that you'd want to both bold and italicize (or underline), both super and subscripting text doesn't make sense.

[wimleers]

Agreed with @quicksketch, and that'd make this far simpler to implement too! Easy for both end users and the maintainers 👍

[wimleers]

Related bug: #15117

[CKEditorBot]

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[quicksketch]

Still interested in this.