Is it another forgotten setting?
Whether this setting is on or not, it's not saved to the project. But it does change the stage size and inadvertently break the project.
It doesn't break the project, but it does make it easier to code and test simultaneously.
i dont see how this breaks the project? it seems to work perfectly fine for me..?
Just as expected, changing the stage size will cause effects on projects that rely on the stage size. It also saves the new stage size but not the flag toggle.
I'm not sure what the purpose is for, or the reason behind existence. That's what one would think, but I'm not totally sure. The UI looks more confusing to use with scripts and sprites mixed up, rather than having them in separate regions.
the video I sent has a project that relies on stage size and it doesn't seem to break, unless I am misunderstanding something
it is probably for demostration purposes. for example, presenting a project and how it works to a class.
Performer mode helped me with longer scripts and editing them while the project was running
If it doesn't seem to break, it's maybe you used absolute coordinates. I sometimes use relative coordinates (which depends on the stage edges), but when I make projects I only use them for neat alignment and I didn't take stage sizes into account. Problems still occur if anything were to interact with edges (like bouncing sprites).
The Turtle sprite initially present in new projects isn't anchored to the center and get pushed to the top-left for some reason.
Can they use normal mode? Unless they want a layout where the scripts are more scattered, it would have been tidier to place scripts in one side and sprites where they belong. Not to mention one has to switch between interacting with sprites and scripts and neither interact with each other.
How?
All I was able to figure out is that it's just that the stage is behind the scripts. You have to switch between layers with the turtle button instead of simply move your cursor between the scripting area and stage.
I see what you mean now, I was confused because my project auto-resized properly.
it gives you more horizontal space, I like it. it's there if someone wants to use it. if you don't like it, don't use it 
I was only asking a question. I was curious why this setting was added for. It doesn't save per project, it changed another setting and left it behind (and potentially would break projects just to enable), extensions don't support it, ...
Those factors made the purpose more obscure.
is this tested? i think it returns back to normal if you uncheck the option. at least that is what it did with the two projects I just tested.
It depends on the project, but I am sure there is at least one type of project that will suffer from this.
this should probably be reported as a bug report
It's for projects that teach how to use the UI. "This area on the left is the palette. Grab a block from there and drag it into the scripting area." I think someone at EDC made it.
But it doesn't save per project. Did they forget to make sure it does?
I don't know, sorry.
No, no, noooo!, Performer mode is for live coding (that's a term, look it up!). It was developed for Snap! by Bernat together with Enric from the Open University of Catalonia and the crew at the CitiLab Barcelona, where they have a vibrant live coding scene in bars etc. It's used both by professional musicians and pedagogues for ... live coding. You should remember those live performances from last Robolot and SnapCon conferences in case you attended.
OK, sorry.