Steps to reproduce:

• Start Designer, start new QMainWindow-based form
• Drop some widget onto central widget
• Set size policy of central widget to "Fixed"
• Note: widget disappears
• Note: The central widget gets a very small size in that dimension

Similarly, loading qtbug43797.ui with Designer or the attached UI loader shows that the central widget becomes too small.

When you use Qt Widget Designer to set centralWidget's sizePolicy to fixed all the existing widgets in centralWidget are hidden and you cannot add new ones.

This is reproducible in any Form based UI e.g. with the default Qt Widgets application template. Just add random widgets to the main window and set centralWidget's horizontal or vertical sizePolicy to fixed.

This is not reproducible in Qt Creator 3.2.2. And tested only on Windows.