rainbow wheel of death during save

I had been running v0.6 of MacDown quite happily for a few weeks, but now whenever I attempt to save changes to an old or new .md file, the rainbow wheel of death is displayed. I eventually have to force quit the application. No changes are saved to the file.

I am at a loss to understand what has changed in my system's configuration that would have caused this type of thing to start occurring.

Start by reproducing the reported hang when saving both old and new .md files in MacDown v0.6. No source files or tests are identified in the report, so trace the save entry point and determine why the application stops responding. Done means saving completes and changes persist without requiring a force quit.

Written by the indexing model from the issue text.