Set proper panel widhts in splitter

[hanastasov]

Closes TFS #266999

Additional information (check all that apply):

• [x] Bug fix
• [ ] New functionality
• [ ] Documentation
• [ ] Demos
• [ ] CI/CD

Checklist:

• [ ] All relevant tags have been applied to this PR
• [ ] This PR includes unit tests covering all the new code
• [ ] This PR includes API docs for newly added methods/properties
• [ ] This PR includes general feature table updates in the root README.MD
• [ ] This PR contains breaking changes
• [ ] This PR includes behavioral changes and the feature specification has been updated with them