This has been a long-standing problem that I hit every so often. It takes a second or two to remember what to do and then I move on. Here is the scenario:
Open a project (or start the IDE that was previously running a project). Close the file(s). Now open a new file that doesn't belong to a project (I have several of these) and try to compile. The Ctrl-F7 Compile menu under Tools is grayed. The reason is that the IDE still thinks the project is open. The workaround is to close the project, which unfortunately unloads the file just opened and then reopen the file again.
I would like to a project automatically closed if the last file in the project is closed. Also closing a project should only close the files that belong to a project.