Odoo’s user interface framework was upgraded from Bootstrap 3 to Bootstrap 4, along with related date, tooltip, popover, and theme components. This keeps the product on a more current design foundation, improves long-term maintainability, and may noticeably affect page layouts and responsive behavior across many apps.

Task: https://www.odoo.com/web#id=49280&action=327&model=project.task&view_type=form&menu_id=4720