When I click on the image attachmenys in this forum, the overlay that pops up is shifted drastically to the right. Often, I can't even see the whole image, because no scroll bars are offered, nor should they be necessary, as I have an Apple 30 inch Cinema Display. I'm including a screen shot for your amusement. In the example, even if I enlarged the window as much as possible, I could not see the whole image:
I'm also using Safari on a Mac, but I'm not seeing the issue you have.
After testing with the same popup (Noel's) I saw somthing that I hadn't realized before.
It appears that Safari will always attempt to center the image depending on the horizontal width of the browser window.
So if I have a wide brower window taking up most of my screen real estate, the popup will be centered and large.
But if I make a narrow horizontal width browser window, it will still center it but the popup is much smaller.
Of course, this is dependent on the original size of the uploaded image, i.e. the original poster's image popup shows as the same size as the original image on the page.
People who know what they are doing (like Noel) upload a more generous size so that clicking on it gives a larger more detailed image which is the whole point of popups to begin with.
Have you have this problem in the past? Does it appear if you use a different browser?
What happens if you "Empty Cache" in Safari, or the more extensive "Reset Safari" (under the Safari menu)
I just tried this in Firefox, and it works normally. So something is affecting the way Safari renders the Adobe forum pages. I'll try disabling my user style sheet in case it's the culprit.
Update: I disabled my user style sheet to no effect. That is not the problem. It contains a few little fixes for sites I use daily, but nothing that affects this site.
I hate a full browser reset, because I don't want to lose all my form values, cookies, etc, but I'll empty cache. I don't think this will help, because the problem survived my upgrade to Mountain Lion, which was a clean install. But I'll try it anyway, because I may have dragged my Library/Safari folder over wholesale.
Update: Emptying cache did not fix the problem. Sigh.
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