When I have two projects open and want to copy a file from one to the other by dragging 'n' dropping it from the file tree, nothing happens.
Still exists in 5.2.0. In the 'Projects' toolview, moving files within a project works (and issues `git mv` commands for a git project); moving them across projects fails silently. In the 'Filesystem' view it doesn't seem possible to move files at all, quite possibly by design.
A possibly relevant merge request was started @ https://invent.kde.org/kdevelop/kdevelop/-/merge_requests/248