In QGraphicsLayout, the whole height-for-width stuff seems to be broken.

The attached program creates a SquareWidget whose sizehint tries to make it a
square. Then this is added a layout.

By passing in the width as the constraint, the square gives the correct
sizehint:

Preferred size of square of width 100 is QSizeF(100, 100)

But passing in the width as the constraint to the layout gives the wrong
sizehint:

Preferred size of layout of width 100 containing a square is QSizeF(100, 20)