https://developer.gnome.org/gtk2/stable/GtkBox.html#gtk-box-pack-start fill
TRUE if space given to child by the expand option is actually allocated to child , rather than just padding it. This parameter has no effect if expand is set to FALSE.** A child is always allocated the full height of a GtkHBox and the full width of a GtkVBox. This option affects the other dimension**
That does not seem to be happening to the hbox containing the textedit in question !
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