The GtkScrolledWindow uses the widget prefered size as a guess as to whether scrollbars are needed or not in the automatic scrollbars case. If we don't report anything for them we typically get it wrong and cause two size allocate calls on the child each time, with different sizes. This (the two sizes speicifically) will cause unnecessary relayouts of the window. So, we just report the current size of the layed out icons as the prefered size. This is somewhat wrong as its depending on previous size_allocation calls rather than the "ideal" size of the widget, but since the ideal size is ignored anyway and just used for this it works well. taken from: https://git.gnome.org/browse/nautilus/commit/?id=fa6e447