I have the latest iteration of FF (build 111) and have been trying to construct an off-canvas menu (following Mansour's great video by the way).

I get as far as nesting the third container and configuring the attributes when, to my dismay and for the eighth time now, find that all the containers suddenly disappear and the project is locked with only the 'grid' tab showing; there is nothing else now in the project. I cannot add any new elements and the only option is to start a new project.
I have followed the same construction route each time - to make sure it is not something that I am doing wrong - and the result is always the same ... the elements disappear and the project locks ( the application doesn't crash but the current project is unusable and has to be scrapped ...... very frustrating).

Has anyone else had this happen?

Select your third container from inspector pane and change it position from absolute to static and add / modify your elements then once you are happy with you design make sure put back the position to absolute.

I don't think so that could be a bug, that the nature of "off-canvas class" as this class intend to push elements far from stage / canvas .

1- As I said in my above post either set third container position to static ( off-canvas class has some pre style which push elements out of stage with -250 Margin as what it suppose to do) default position of every elements then add your 4th container. ( As what I did in off-canvas tutorial )

2 - Or before applying off-canvas class to third container, add your 4th container then apply the off-canvas class to 3rd container.