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Odd. I see this as well, but only sometimes — sometimes, when I close an untitled document, I get the “Do you want to keep this new document?” dialog as expected, and if I click “Delete”, it’s deleted from iCloud as expected.

However, sometimes (particularly if I leave the untitled document open for a long time), I get no dialog on closing the document and the untitled document remains stored in iCloud.

Does the app have any control over this behavior?
I see this occasionally as well, unfortunately I likewise haven’t seen any interesting Console messages or the like. The only indication I get that autosave has stopped working is that the “Edited” decoration doesn’t show up in the window’s titlebar. Typically the issue only occurs after Textastic has been running for a few days. I can cmd+s to manually save the file, and a quick cmd+opt+q and relaunch brings autosave back.

My files are just on the local filesystem (I don’t have Dropbox installed), but they are also typically in git repositories. I’ll keep an eye out for anything else that might help nail this down.