Fix #6409 - Datatable: ColumnGroup refactoring

[Rapster]

Fix #6409
Fix #9854

[tandraschko]

before you work on a PR which refactors such a important feature, we should discuss what you refactor and if we are all okay with it; especially PrimeTek and if we can also be backward compatible

[Rapster]

PrimeTek has already been pinged regarding this, and no answer. Community is aware of it, and also I'm trying to make it backward compatible as much as I can. If you have any feedbacks, please feel free to share in the original issue

[nimo23]

@Rapster I created #9572 and I wanted to delve into the DataTable-component model to solve this problem. However, I see that you have already made many changes here in this PR, also in relation to #9572 (e.g. you did a lot of improvements in relation to frozenColumn, etc.), so it is not appropriate for me to delve into the "still old" code of the DataTable-component at this time.

So it would be cool if you maybe noticed how you can solve #9572 alongside with the code you refactored - maybe now it's much easier (for you) to solve #9572 with your refactored code.

You already have a deep understanding of the DataTable-component that I don't have. With the many code improvements in this PR, it might also occur to you to fix #9572 at the same time.

Perhaps #9572 is now much easier to fix than with the current "old code" of the DataTable-component. Anyway, I've brought this to your attention and maybe, by the way, you'll see a solution right away :)

[Rapster]

Don't wait for this PR to be merged, I don't think it's gonna happen anytime soon (few things must be done before having this merged)

[nimo23]

I don't think it's gonna happen anytime soon (few things must be done before having this merged)

No stress. Maybe one day while editing this PR you will casually spot the cause of #9572. Maybe then the problem can be solved at the same time. Just a suggestion. I just wanted to draw your attention to #9572, since you are changing a lot in the Datatable anyway.

[Rapster]

@primefaces/primefaces-committers I'm actually thinking of not keeping the backward compatibility here, it's a lot of hassle to make it so for a feature which is not so much used... Hence the importance to make 15 final in 6 months

[melloware]

@Rapster i agree that to fix this correctly it needs to be refactored as you have mentioned we just have to make the migration guide extremely detailed. I think more people use this feature than we think but who knows! 😄

[Rapster]

No you're right, I had look on issues, and col group are used more than I thought... But it doesn't change my previous statement on how tricky it is to make things backward compatible. Hopefully, migration should be easy for people using nested grid with flexgrid (just to name one example)