I'm running OSX 10.6.8. I've uploaded two screen shots (42% and 50%) within a zip file to: http://www.mediafire.com/?e65ra2us8a3f4t8

I can post here if that's easier for you. I thought the 450 px width limit imposed here would make the image difficult to interpret.

The problem is to do with Preferences > Interface > Open Documents as Tabs. When that option is ticked, I get the strange behaviour in the scroll bar. When that option is unticked, the scroll bar works as it should, but new images open in a new window.

I'd like new images to open as tabs and each tab show the size of the image in the scroll bar correctly.

I've also noticed that Preferences > General > Zoom Resizes Windows doesn't work when tabs are selected. The window always stay the same size, and I assume, cause the incorrect bar size in the scroll bar.  Maybe Photoshop thinks the window is resizing when it isn't.

In CS4 it bothered me that if I resized one tabbed image (because I had chosen to Resize Windows), all the other tabbed images would also have resized windows. At least I think that's what happened. Maybe the designers tossed that idea and a tabbed window now always stays the same size, but they forgot to alter the scroll bar size.

Anyway, I think my CS6 has a fault. This is what I did:

1. Turned off Open Documents as Tabs and opened a file. It opened as 50%. Horizontal scroll bar = 440 mm, no size bar showing. I assume that means the size bar is also 440mm.
2. Zoomed in to 100%. The size bar was 220 mm in length, which is what it should be (i.e. I am only seeing half the image, so the bar shows 0.5 x 440 = 220mm).
3. I checked at 200% (size bar = 110mm = correct); and 400% (size bar = 55mm = correct)
4. Closed image, set Preferences > Interface > Open Documents as Tabs, and opened the same image. Everything the same as in Step 1.
5. Zoomed to 100%. Size bar should be 220mm, but is actually 111mm.
6. Zoomed to 200%, Size bar should be 110mm, but is actually 74 mm.
7. Zoomed to 400%, Size bar should be 55mm, but is actually 44 mm.

Now the interesting thing is, that last size sequence is non-linear. It looks like PS is doing calculations based on incorrect numbers but in a convoluted way.

Can anyone reproduce these results?

If it's working as designed, why are there two different implementations – one when tabs is on, and a different one when tabs is off?

You should know that it's been this way for many major version releases.

That statement is not correct. I had never noticed problems with scrolling in CS4 or CS5. I often use the bars, when zoomed into an image, to jump to other parts of the image, particularly when comparing two versions of the same image for small differences in sharpness or contrast. In CS4, jumping never went past the image.

To prove that, I just reinstalled CS4, and voilà – the bars work correctly. When I jump, I never see grey, I only see image.

To prove it again, I just reinstalled CS5. Scroll bars work as they should.

But in CS6, when I jump to the extremities I often get a small part of the image and the rest of the window is a huge grey area. Since when do users want to see grey instead of image? Total hash of what is supposed to happen.

This is bug in CS6. It infected Phostoshop a few months ago. If it was designed to be that way, then it's a flawed design that should be changed.