Reproduction steps:
1. Create a new project
2. Create UI Document (Create > UI Toolkit > UI Document)
3. Double-click newly created Asset to open UI Builder window
4. Click “+” button in the Stylesheets section and select “Create New USS”
5. Click on :root Selector
6. Add a new Variable in the Inspector (Styles > Variables)
7. Change Variable name to “-“, press enter and observe

Actual result: Variable name becomes empty
Expected result: “-“ is displayed as Variable name

Reproducible with: 6000.2.8f1, 6000.3.0b6, 6000.4.0a2
Couldn’t reproduce with: 2021.3.57f1, 2022.3.67f1, 6000.0.60f1 (No :root Selector)

Reproduced on: Ubuntu 24.04, macOS Sequoia 15.4 (M4)