How to make those modern Gtk.MessageDialogs

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I believe various applications use this style, so there must be something obvious I’m missing. I can’t replicate this in glade with the GtkMessageDialog, which is apparently what this is made of according to the Gtk Inspector

This is how the one from glade looks like:

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ah, setting layout_style of the Gtk.ButtonBox to expand. It’s displayed incorrectly in glade until the preview is loaded