When you have nested layouts and widgets, invalidating the size of one item doesn't cause all the layouting to be redone, but it's done in piecemeal.

There was a bug for this months ago, with the same attached test case, but I cannot find it anywhere, and some of the related bugs seem are missing attachments and missing comments.

If you run the attached program, you can see widgets inside of widgets. The inner most widget's preferred size changes, but visually you see that it grows larger than its parent.