Discard additional lines upon inserting when AcceptsReturn=false

[markl65536]

What does the pull request do?

It changes text insertion into TextBox with AcceptsReturn==false so that behavior is the same as in WPF.

What is the current behavior?

Upon insertion of multiline text into TextBox with AcceptsReturn==false the TextBox becomes multiline. It is currently impossible to prevent a TextBox becoming multiline by user input (e.g. by pasting from clipboard).

What is the updated/expected behavior with this PR?

Upon insertion of multiline text into TextBox with AcceptsReturn==false, only the first line will be inserted and any additional lines will be discarded.

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

TextBox.RemoveInvalidCharacters was renamed to SanitizeInputText and extended to handle the multiline scenario.

Fixed issues

Fixes #5600

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0043496-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[maxkatz6]

cc @Gillibald

[Gillibald]

 var linebreakEnumerator = new LineBreakEnumerator(text.AsSpan());

 while(linebreakEnumerator.MoveNext(out var linebreak))
 {
     if (linebreak.Required)
     {
         text = text.Substring(0, Math.Max(0, linebreak.PositionMeasure - 1));

         break;
     }
 }

[Gillibald]

This is the way you should look for line breaks

[markl65536]

I will amend my PR.

[markl65536]

The tests fail on Windows because LineBreakEnumerator does not handle the line breaks as expected on Windows (I develop on Linux and tests pass fine there). Windows uses CRLF as line terminator but LineBreakEnumerator leaves a trailing CR. Referring to Unicode Documentation this may be according to spec, because a CR does not cause a line break when followed by a LF, but it is probably not what we want for TextBox.

I am at a loss here. Should I add additional code for "cleanup" of extraneous trailing CRs? This is kinda ugly but I do not see a better way when using LineBreakEnumerator. Any suggestions?

(Notice that in deviation to the suggestion of @Gillibald I am using PositionWrap instead of PositionMeasure. PositionMeasure would trim all trailing whitespace. That would solve the CR-problem but it would kill trailing space.)

[Gillibald]

I have adjusted the logic to only look for line-break characters

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0044651-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]