I'm sometimes working at a customer site where they use Microsoft TFS for version control. TFS implements pessimistic locking and makes all files read only on a file system level, *unless* they are "checked out". So, most files here are read only. That's fine for opening the file in SQL Developer and running its contents (e.g. a migration script or a package spec / body). But unfortunately, once SQL Developer has an open worksheet containing a readonly file, it's no longer possible to use Windows file drag and drop into SQL Developer. I first have to open a new writable worksheet, and only then I can drag and drop the file.
The same is true when starting up SQL Developer, when there is no worksheet at all. In that case, I cannot drag and drop a file into SQL Developer, I have to create an empty worksheet first.
To illustrate the problem, I've created a short screencast:
