[OS X Emacs] Aquamacs 2.1: transient-mark-mode going berserk on me

[Christian Kirsch]

Hi,

I'm using Aquamacs 2.1 on Mac OS X 10.6.4. The editor is now quite often
going into some kind of automatic transient-mark-mode: I position the
mouse somewhere in the text and click to move the cursor to this
position. If I then press <down> (without shift!), the whole text
between the last mouse position and the new cursor position is
highlighted. It behaves as if I had marked marked it with S-<down>. I
can temporarily turn off the highlight with C-g, but if I move point
again with one of the cursor keys, I get highlighted text again.

Can anybody tell me what I'm doing wrong here and how I can revert to
the previous behaviour, i.e. select text only with shift pressed during
cursor movement?

And no - caps lock is *not* on. It's disabled for this keyboard.

Thanks in advance
Christian