The Python code below will create a window with a fixed layout containing a container with red background and a label inside. The label is set to ellipsize its content if it is longer than the size of the label.

In GTK 2 the window would have been 100px wide when executing this and this is also what I'd expect.

What actually happens in GTK 3 is that the label and the surrounding event box are 100px wide as requested but the window has the size it would need to have to display the whole text.

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from gi.repository import Gtk, Gdk

wnd = Gtk.Window()
box = Gtk.EventBox()
fixed = Gtk.Fixed()
lbl = Gtk.Label("Long label..with lots of text.. foo bar baz")
wnd.add(fixed)

box.add(lbl)
box.modify_bg(0, Gdk.Color(0xffff, 0, 0))

fixed.put(box, 10, 10)
lbl.set_size_request(100, -1)
lbl.set_ellipsize(1)

wnd.set_size_request(100, -1)

wnd.show_all()

Gtk.main()