Fix nth child selector in data grid header

[appel1]

What does the pull request do?

Attempt at getting nth child selectors to work with data grid column headers.

What is the current behavior?

If you add or remove columns :nth-child and :nth-last-child applies to the wrong element.

What is the updated/expected behavior with this PR?

That nth-child and nth-last-child applies to the expected elements after adding or removing columns.

Fixed issues

Fixes #14695

[timunie]

@appel1 for debugging, you can also use src/Samples/Sandbox. You may need to add a reference to DataGrid project. Just fyi.

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0047602-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[appel1]

@timunie Thanks for that, made things easier.

But why was the PR closed when I added another commit to it?

[appel1]

Not sure if this is the correct approach, but as far as I can tell it works after these changes.

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0047608-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0047630-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[appel1]

Realized that taking visibility might be a bad idea. If you change visibility in the selector for example.

I'll check how this is handled elsewhere in Avalonia and update the PR.

I think nth child in CSS counts elements even if they are hidden.

[maxkatz6]

Yes, nth child does count elements if they are hidden. The same goes with avalonia nth child, IIRC.

[appel1]

What about the hidden DataGridFillerColumn? Should that also be taken into account? So that nth-last-child(1) matches the filler column.

[jmacato]

@cla-avalonia recheck

[cla-avalonia]

• All contributors have signed the CLA.

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0047774-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[appel1]

@cla-avalonia agree