Allan,
Many thanks - that does fix the symptoms, but I remain puzzled about the original nature of the problem. For total clarity, if I put the old "projectDirectory" line back, then these are my steps to reproduce the problems:
(0. Close all windows in open TextMate, then quit.)
1. Open TextMate. An empty window appears. Close it.
2. File -> Open Recent -> "docs" (a subdirectory of "Git"). A window with an untitled tab appears, with " — untitled" as the window title. CPU usage goes up to 100% (and stays there).
3. Select a .tex file from the project window. Window title is now "docs — myfile.tex", as intended. CPU usage goes up to 200% (and stays there). So the problem seems to spread beyond untitled files.
4. Press ⌘R. Nothing happens.
With the modified .tm_properties, everything is fine. Is the problem with the .tm_properties file blocking the thread that would be used to run the command?
I was aware of the empty part of the window title for untitled files, but it never seemed to cause a problem before, and since I don't work with untitled files for long I wasn't bothered about fixing it from an aesthetic point of view. Either way, it seemed like a harmless thing (until now!).
All the best,
Jon