Advanced Search affected with Confluence 7.4 when configuring viewport on root level

Steps to reproduce using Confluence 7.4

1. Configure Confluence to use no context path.

2. Configure viewport with path '/' and any standard theme Web Help or Help Center

3. Access some content through regular Confluence UI.

4. Type some text in search window in upper right hand corner and click Advanced Search

5. Confluence advanced search UI is displayed or error message is thrown.

Expected Result

Confluence advanced search UI is displayed without any UI modification and without any error message.

Actual Result

Viewport theme is "modifying" Confluence advanced search UI:

Open

Open