Created attachment 328811 [details]
testcase

In the attached testcase there's two labels (it can be any other widget really) attached through a GtkSizeGroup. One of the labels is shown, and the other is initially hidden, but can be toggled visible/invisible.

As GtkSizeGroup uses to work, I'd expect the grid to initially grant only space for the first label, but the label should be requesting enough height to avoid global height changes when the second label is shown. When the second label is shown, the grid should only require changes in width to accommodate the extra widget which should be as wide and high as the other label.

However, when running the testcase, it seems the first label gets just enough space to be visible itself, and the second label gets a similar allocation, but not accounting for the height overall, so the second label is rendered outside its allocation.

Even funkier, making the inspector appear makes the widgets snap into the right sizes.