This is from the release notes for Mendix 6.7.0:
We removed the Is close button property that was only available on data view control bar buttons. Previously, you had to always mark one button on a page as the close button, even on content pages. The Is close button property was used to configure the behavior of the pop-up window close button (that is, the cross in the top-right corner). When the cross was clicked, it would then act as if the marked button was clicked.
You can now configure the behavior of the cross using the Close action property of the page. This property is only available on pages that use a (modal) pop-up layout. The default behavior of the cross is to roll back any changes and to close the pop-up window. We hope this behavior is suitable in most cases. If you want to customize the behavior of the cross, you can still point to a button on the page. If the selected button is not available at runtime, the cross will revert back to the default behavior.