filepath copy button in the header when a file is open

Title explains most of it.

Would be nice to be able to copy the filepath when working on the project to be able to open it another editor if working on a larger set of changes

Start at the header when a file is open, then trace how the current filepath is represented and whether the project already has a copy action. Done means the header exposes a copy control for the open file's filepath and copying it produces the path needed to open that file in another editor.

Written by the indexing model from the issue text.