[MusicXML] Rhythmic slash notation badly handled

Issue type

Import/export issue

Description with steps to reproduce

1. Enter some chords
2. Toggle rhythmic slash notation
3. Export to MusicXML
4. Import exported file

Currently chords in slash notation are exported with invisible noteheads except for one which is marked as slash notehead (because MuseScore sets all noteheads except one to invisible). This should be exported by using a <slash> element at the beginning and the end of a section with rhythmic slash notation.

Supporting files, videos and screenshots

Entered notation

After ex- and reimport

Test files

rhythmic_slash.zip

What is the latest version of MuseScore Studio where this issue is present?

latest nightly build

Regression

No.

Operating system

macOS 15

Additional context

Wrong import of (properly encoded) slash notation was reported in #16216.

Checklist

• This report follows the guidelines for reporting bugs and issues
• I have verified that this issue has not been logged before, by searching the issue tracker for similar issues
• I have attached all requested files and information to this report
• I have attempted to identify the root problem as concisely as possible, and have used minimal reproducible examples where possible

Start by opening rhythmic_slash.zip and reproducing the MusicXML export/import workflow described in the issue. Compare the exported XML and the reimported notation; done means rhythmic slash sections are represented with slash elements at their boundaries and survive the round trip correctly.