OK. Thanks for the suggestion.

The size hint is already growing on date3 when the longer display format is set but, even though the sizePolicy on the editor is (Minimum, Fixed) (seems to be the default), the editor visual stays smaller than the hint.
Qt Code: 
  1. sizeHint() before =  QSize(90, 21)
  2. sizeHint() after =  QSize(114, 21)
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It seems that I need to manually call QWidget::adjustSize() to have the visual size change. I expected the updateGeometry() method to alert the layout but this does not seem to happen. Judging from Designer-generated code it expects this too: no call to updateSize() is made after changing the display format.