Fix broken pre-edit markedText on iOS and solve #17523

[Arktische]

What does the pull request do?

Fix #17523.

What is the current behavior?

Can't type any multisyllable language including Chinese, Korean etc.

Bug Descrption

The CombinedSpan3.CopyFromSpan implementaion is wrong.

    static void CopyFromSpan(ReadOnlySpan<T> from, int offset, ref Span<T> to)
    {
        if(to.Length == 0)
            return;
        if (offset < from.Length)
        {
            var copyNow = Math.Min(from.Length - offset, to.Length);
            from.Slice(offset, copyNow).CopyTo(to);
            to = to.Slice(copyNow);
            offset = 0;
        }
        else
            offset -= from.Length;
    }


    public void CopyTo(Span<T> to, int offset)
    {
   // the offset is not updated, should use ref here
        CopyFromSpan(Span1, offset, ref to);
        CopyFromSpan(Span2, offset, ref to);
        CopyFromSpan(Span3, offset, ref to);
    }

Another bug occurs in IUITextInput.TextInRange

        string IUITextInput.TextInRange(UITextRange range)
        {
            var r = (AvaloniaTextRange)range;
            var surroundingText = _client.SurroundingText;
            var currentSelection = _client.Selection;
            Logger.TryGet(LogEventLevel.Debug, ImeLog)?.Log(null, "IUIKeyInput.TextInRange {start} {end}", r.StartIndex, r.EndIndex);

            string result = "";
// wrong indent here, it will directly fallback when _markedText is empty or null
            if (string.IsNullOrEmpty(_markedText)) 
                if(surroundingText != null && r.EndIndex < surroundingText.Length)
                {
                    result = surroundingText[r.StartIndex..r.EndIndex];
                }
// this `else` is paired with the second `if`, not as expected
            else
            {
                var span = new CombinedSpan3<char>(surroundingText.AsSpan().Slice(0, currentSelection.Start),
                    _markedText,
                    surroundingText.AsSpan().Slice(currentSelection.Start));
                var buf = new char[r.EndIndex - r.StartIndex];
                span.CopyTo(buf, r.StartIndex);
                result = new string(buf);
            }
            Logger.TryGet(LogEventLevel.Debug, ImeLog)?.Log(null, "result: {res}", result);
            return result;
        }

What is the updated/expected behavior with this PR?

Able to input Chinese or other multisyllable languages using system IME on iOS

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

Implementions are simple as commits showed.

Checklist

Since this fix is simple, those steps could be omitted

• 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

Breaking changes

None

Obsoletions / Deprecations

None

Fixed issues

#17523

[maxkatz6]

LGTM

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0053627-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[kerams]

Many thanks, pinyin input now works great. No change in Korean though, which probably points to a different underlying problem.

[Arktische]

@cla-avalonia agree