This function is only used by gtk.Container subclasses, to assign a size and position to their child widgets.
In this function, the allocation may be adjusted. It will be forced to a 1x1 minimum size, and the adjust_size_allocation virtual method on the child will be used to adjust the allocation. Standard adjustments include removing the widget’s margins, and applying the widget’s halign and valign properties.
For baseline support in containers you need to use Widget.sizeAllocateWithBaseline instead.