window-list: Update keyboard position on fullscreen changes

The visibility of the window list changes when a monitor enters or
exits fullscreen, so we should update the OSK position accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=723693

extensions/window-list/extension.js

@@ -786,6 +786,7 @@ const WindowList = new Lang.Class({
 
         this._fullscreenChangedId =
             global.screen.connect('in-fullscreen-changed', Lang.bind(this, function() {
+                this._updateKeyboardAnchor();
                 this._updateMessageTrayAnchor();
             }));