When odd things like this happen it's always worth a shot at trashing the Lr Preference file? You'll find details on location at http://forums.adobe.com/thread/358041?tstart=0
Also, worth check that your mouse settings in Apple System Preferences weren't to something weird during the upgrade to Mountain Lion
No go. Trashed the preference files before posting here. I didn't upgrade to Mountain Lion; it came on the computer preinstalled so all the installations are fresh. Also, this is only a problem inside LR, which says to me that the problem is LR not the operating system. By the way, I am using the trackpad to navigate LR and I turned off all gestures to see if they were a problem--same behavior.
OK, I think I can trigger what you're seeing. If I click to zoom then drag the mouse even a fraction then release the image zooms back out. Once it starts doing this it's very difficult to stop. This behaviour also occurs in Lion but it isn't as easily triggered.
On windows, at least, the spacebar also will toggle zoom mode. It's an easy keyboard shortcut to use, and it might work as a workaround until the situation gets resolved by some future update.
Also found that keeping the left mouse key depressed for a second or so is enough to trigger behaviour. I've submitted a bug report.
The Spacebar also zooms on the Mac. It is just more convenient to use the trackpad to scroll through the images and zoom. However, it isn't that much more convenient for me to spend a lot of time on. I will wait for a future version to see what happens and in the meantime use the spacebar. I just thought that someone might have an easy answer. Thanks everyone.
I always thought that's by design.
Click-hold-release, as opposed to a normal (fast) click, zooms in and out,
just like click-and-drag.
I have no problems single-clicking reliably on my machine.
2012/7/31 Ian Lyons <firstname.lastname@example.org>
Re: Won't stay at 1:1 mag
created by Ian Lyons <http://forums.adobe.com/people/Ian+Lyons> in *Photoshop
Lightroom* - View the full discussion<http://forums.adobe.com/message/4589842#4589842