GTK3 Resizable Image prevent window restore size

Question

I have written a Class that extend Gtk.image to automatically fit the parent size based on PyGTK: How do I make an image automatically scale to fit it's parent widget? but with some improvements and corrections.

import gi
gi.require_version('Gtk', '3.0')
from gi.repository import Gtk, GdkPixbuf, GLib

class ImageEx(Gtk.Image):
    pixbuf = None

    def __init__(self, *args, **kwargs):
        super(ImageEx, self).__init__(*args, **kwargs)
        self.connect("size-allocate", self.on_size_allocate)
        self.props = super(ImageEx, self).props
        self.props.width_request  = 100
        self.props.height_request = -1
        self.props.hexpand = True
        self.tempHeight = 0
        self.tempWidth  = 0
        self.origHeight = 0
        self.origWidth  = 0


    def set_from_pixbuf(self, pixbuf):
        self.pixbuf = pixbuf
        self.origHeight = pixbuf.get_height()
        self.origWidth  = pixbuf.get_width()
        self.tempHeight = 0
        self.tempWidth  = 0
        self.on_size_allocate(None, self.get_allocation())

    def calculateHeight(self, desiredWidth):
        return desiredWidth * self.origHeight / self.origWidth

    def on_size_allocate(self, obj, rect):
        if self.pixbuf is not None:
            desiredWidth = min(rect.width-10, self.origWidth)
            if self.tempWidth != desiredWidth:
                self.tempWidth = desiredWidth
                self.tempHeight = self.calculateHeight(desiredWidth)
                pixbuf = self.pixbuf.scale_simple(self.tempWidth,
                                                  self.tempHeight,
                                                  GdkPixbuf.InterpType.BILINEAR)
                GLib.idle_add(super(ImageEx, self).set_from_pixbuf,
                              pixbuf)

I use it in a Gtk.Paned, it works correctly, I can resize the panel, and the window, but if I expand the window I can't restore it with the window button but if I move the window, it restores correctly.

How can a so inconsistent state exist? How can I correct it?

Thank you