[OS X TeX] TeXShop closes file or switches to wrong font upon external modification on 10.8

[Ingmar Steiner]

Dear all,

I use TeXShop on OSX 10.8.3, and Git to manage my LaTeX source code. 
Since a recent TS upgrade (now using 3.18), I observe buggy behavior 
whenever I externally modify a .tex file whose source is open in TS 
(e.g., I switch to a different branch in my Git repository, with a 
different version of the .tex file, or I decide to discard a hunk of 
modified lines in the .tex file).

When I do this, I observe one of two apparently random symptoms, the 
second more disruptive than the first:

1) The source editor in TS switches to a variable-width, sans-serif 
font. I've configured it to use Monaco 12 (on a non-retina machine) or 
Menlo 12 (on a retina MBP).

2) The .tex source editor, console, and preview window simply close. 
Note that TS is still running in this case.

Closing the .tex file and opening it again restores the expected behavior.

I should mention that I've disabled Auto-Save, Versions and its ilk in 
OSX by setting

   defaults write -g ApplePersistence -bool no

Before the TS upgrade, I would simply hit CMD-U to revert the .tex file 
in the source editor after any external changes to the file, and get on 
with my work, but now I have to reopen it every time and navigate back 
to the line I was working on. Extremely annoying.

Does anyone have an idea what triggers these bugs, and how I can work 
around them?

Thanks, and best wishes,

-Ingmar