leaflet: Unsatisfied needed alloc generates a critical

Here is the error message:

(example:2): Gtk-CRITICAL **: 09:42:18.043: gtk_widget_draw: assertion '!widget->priv->alloc_needed' failed

To reproduce it you could:

• have a leaflet with a stack switcher and a non-homogeneous stack,
• have the leaflet unfolded but on the verge of being folded,
• select a stack child that is larger than the current one so the stack make the leaflet fold itself.

I don't manage to reproduce the issue consistently, if you find a good way to do so please let me know.