When the statusbar has a resize grip, the statusbar's size-allocate function shrinks the statusbar's label accordingly so it doesn't overlap with the grip.

Now consider the case that the programme replaces the statusbar's frame's child (the label) with a container that he packs the label (and some other things) into. [The use case for that is to make it so that the statusbar's frame extends around the whole statusbar like it does when it just contains the label; if you instead just pack other children directly into the statusbar you get missing bits of frame which look ugly.]

In this case, the size-allocate should shrink that container instead of the label.

I know it's not really nice to poke around in widgets' internal details, but I hope you'll conside the following patch which is functionally completely equivalent to the current code: it just replaces statusbar->label with the frame's child (gtk_bin_get_child) as the widget to shrink.