%TRUE if invisible widgets are ignored.
Deprecated: Measuring the size of hidden widgets has not worked reliably for a long time. In most cases, they will report a size of 0 nowadays, and thus, their size will not affect the other size group members. In effect, size groups will always operate as if this property was %TRUE. Use a #GtkStack instead to hide widgets while still having their size taken into account.
2.8
Returns if invisible widgets are ignored when calculating the size.