I dont see any reason for this. Why is 666 on a file not enough?
Textastic uses the NSDocument architecture of macOS to open and save documents. This enables features like AutoSave, Versions, iCloud Drive and more.
When a file is saved, it is saved at a temporary location and then moved into the target folder. This requires write permissions for the folder.
You get the same permission dialog when trying to open a file in a read-only folder in TextEdit, so this is really a limitation of NSDocument.
OK, I never used TextEdit for anything but nothing. ;) It's still inconvenient though.
Customer support service by UserEcho