Comment 18 for bug 1637180

this is the easiest way to fix the issue (not perfect though, dialog box looks slightly different afterwards):

--- UpdateManager.py.orig 2017-07-20 22:23:11.643707331 +0200
+++ UpdateManager.py 2017-07-20 22:24:03.048405983 +0200
@@ -88,9 +88,10 @@
         self.set_icon_name("system-software-update")
         self.set_position(Gtk.WindowPosition.CENTER)

- # Keep window at a constant size
- ctx = self.get_style_context()
- ctx.connect("changed", lambda ctx: self.resize_to_standard_width())
+ # commented as this causes high cpu load
+ ## Keep window at a constant size
+ #ctx = self.get_style_context()
+ #ctx.connect("changed", lambda ctx: self.resize_to_standard_width())

         # Signals
         self.connect("delete-event", self._on_close)

I suspect that the resize_to_standard_width method causes a changed signal, which causes it being called again (and again and again ...).

Guess this should be pretty easy to fix for someone who has a clue about gtk (= not me).