i agree that it is not clear when you are looking at a sketch that is yours vs one that is not. i'm open to discussion and ideas about how to make this clearer!

There can be 2 courses of action according to me, 1. The option should be blurred out and on hovering on the disabled button, there should be a Tooltip (in language of Material Design) displaying that the user should clone the sketch first.
2. On the other hand, one can display the Modal on clicking the Add File/Add Folder menu that the person should clone the sketch first.
Both have relative drawbacks over the other. Using method 1, the person has to hover over a disabled option to reveal the Tooltip, which is not intuitive, while using method 2. there are more screens a user has to click through (which may not matter in this case, as the user gains knowledge about the possible course of action immediately after opening the modal).
Method 1 can be optionally extended by displaying a ? over the disabled menu option to let the user know to hover on it, (to display the reason for it being disabled), but the design would become cluttered, especially in the sidebar component.

Since there are no tool tips anywhere in the menu as it exists, it seems like a better solution would be to use an alert / modal for notification. It feels more immediate and would require very limited scope to implement.

as a third option, we could simply disable menu items based on the relationship of the sketch to the owner. eg. if you don't own the sketch, don't show the add file/folder.

yet another option (I present this with healthy skepticism) is to add a duplicate button to the front of the UI if you are looking at a sketch you dont own.

@bmoren According to me adding Duplicate Sketch button in the front of UI would not convey the fact that one needs to duplicate it in order to add files or folders(and would give a more general Fork me on GitHub sense of feeling). If we can find someplace to add the advantages of Duplicating the sketch, it would be much better. Also I agree with your idea of dropping the tooltips altogether because it doesn't go with the present design framework of the editor. Thus modal seems to be a viable option albeit it's disadvantage (interrupted UX flow for something which is not urgent)

as a point of reference, this is handled on CodePen projects by making everything read-only, and communicating that with toasts:

I'd love to see this implemented (I might have time to work on it during next semester). During this semester, I've again had students get stuck here frustrated they couldn't upload files only to realize that they'd not duplicated the sketch yet. So I still feel like its something that would improve the overall experience!

I'm going through old issues, closing out stale ones, etc. Looking back at this issue, I was curious to know how Glitch handles this, and they simply make it so you can't edit other projects unless you "Remix" it. If you try to, this little toast appears:

I think this is the way to approach this issue!

Hey, @catarak!!
I'd like to work on this issue.

Hello! I am a new comer to open source and trying to do my first pull request. So I need a little help. Is there a way to know current users id also including the case when user is not logged in?

Oh! actually I checked your github folk to but didn't found anything but now in some days I will get it. By the best of luck for your pull request. Hope community founds it useful.

Hey @catarak,
I've done this like 10 days ago and am waiting for your reply!
So I'm posting a video of the work I've done

I thought using the toast functionality which we use in many other cases is the way to go.
Share your thoughts!!

Hey @catarak, please look into this

First time contributing to open source. Is this issue unresolved? if it is I would like to work on it