Redmine

When the Gantt page first opens in mobile view, its column resize handles are enabled. They should be disabled in mobile view.

Steps to reproduce:

1. Set the browser window width to less than 900px.
2. Open a project's Gantt page.
3. Move the pointer to the right edge of the subject column.

The cursor changes to a resize cursor but resizing does not work. Resizing the browser window disables the handles as expected.