Which of the following is true about Mendix popups?

Mendix popups are designed to improve user experience by being adaptable to different types of content. The statement that popups adjust size automatically based on their content is accurate because they are meant to fit the information being displayed. This adaptability ensures that all content is visible without unnecessary empty space, providing a more polished and user-friendly interface.

In contrast, the other statements do not hold true within the context of Mendix's design principles. While certain layouts might employ fixed dimensions or layouts, popups are optimized for flexibility, allowing for better presentation of dynamic information. Therefore, automatic size adjustment is a crucial feature that supports a responsive design approach within the Mendix framework.