Fix SlicedStream.Seek(offset, SeekOrigin.End)

[pjt33]

This fixes #13604

The offset _from is applied in set_Position, so applying it also in Seek mispositions the stream. ZipArchive exposes the problem by seeking to the end to read the table of contents.

What does the pull request do?

Fixes a double-offset in SlicedStream which should only be a single offset. Resolves #13604

What is the current behavior?

SlicedStream.Seek(offset, SeekOrigin.End) will almost always seek to an incorrect position.

What is the updated/expected behavior with this PR?

After calling stream.Seek(offset, SeekOrigin.End) it will be the case that stream.Position == stream.Length + offset).

How was the solution implemented (if it's not obvious)?

Remove an offset in Seek which is also applied when Seek calls set_Position. Note that this was only incorrect for SeekOrigin.End: for SeekOrigin.Begin and SeekOrigin.Current the offset was (correctly) not applied.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Fixed issues

Fixes #13604

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0055141-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[pjt33]

@cla-avalonia agree

[MrJul]

LGTM, thank you! I took the liberty of adding some related unit tests (we had none for SlicedStream).

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0055163-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]