https://www.dotnetzone.gr:443/cs/blogs/birbilis/archive/2015/11/28/gotcha-wpf-usercontrol-sizechanged-event-not-firing-at-resizing.aspxUseful to know: If you set the Width and Height on the UserControl though, you have set a fixed size and thus even if its parent tool window changes size, the UserControl never will.  You should not set Width and Height on the UserControl if youelCommunityServer 2.1 SP3 (Build: 20423.1)