The height for width computation of the QStackedWidget seems wrong.
If you add a QLabel with A LOT of text (and with long lines), then the QStackedWidget won't be resized at the correct size.
How to reproduce:
open designer
create widget
add a QStackedWidget into the widget
add a QLabel into the QStackedWidget ( a lot of text with long lines, wordwrap on).
Preview
Play with the resize of the widget
-> the QLabel doesn't show all the text when the width is small.

See .ui file as an example.