Symptoms

When opening up a page in the page Designer or Branding Editor, it starts to load but then refreshes with just the page and no Designer or Branding Editor. 

This behavior is sometimes referred to as Frame Busting. Here is more general information on Frame Busting.

Release

ALL

Cause 

A widget on that page which is busting out has some code to redirect or replace the top.location. 

Here is one example of how frame busting can be achieved, but it may be done in other ways. 

if($window.location != top.location){
top.location = $window.location;

Resolution

To identify which widget is causing this you can remove them one at a time and see where the frame busting stops. Then inspect the client side code of that widget for what is doing the redirect and remove it when using the designer or branding editor. 

You can also search all widget's client scripts for top.location or $window.location to see if any of them are using that line of code which could cause frame busting.