It looks like you're on a very small display -- what are its dimensions? Does it meet the minimum requirements?
Some things to try:
1. Expand all of the other panels.
2. Try hiding and then showing all of the develop panels using Windows > Panels > Histogram, Basic, etc.
3. Reset LR's preferences: Resetting the lightroom preferences file - updated - Lightroom Forums. Sprinkling that magic fairy dust fixes many issues when the UI decides to go wonky.
I'm on a 13" notebook so the display is small; however, the scroll bar was visible before. I've reset the preferences, but that made no difference.
Although I don't think the size of the display is the issue, you did give me an idea. I closed the left panels (e.g., navigator, collections, etc.) to increase the overall area on the right side of the screen. Unfortunately, that didn't work either.
Next I dragged the right panel to make it wider and noticed that the scroll bar would reappear then disappear. Okay, so now I'm thinking I may have accidentally made the right panel too narrow for the scroll bar to be visible since making it wider made the scroll bar reappear. But here's the weird part...making the right panel too wide also made the scroll bar disappear. I had to size the right panel just right for the scroll bar to remain visible.
Glad you got it visible. It smells like a bug handling small displays -- what are the pixel dimensions of the screen?
Oops, the screen is actually a 11.6" with a resolution of 1366x768. I agree that it's probably a bug. You could logically reason that making the panel too narrow could cause the scroll bar to disappear, but you wouldn't think that making it too wide would do the same thing. Btw, the left panel and its scroll bar has no problems when I adjust the size.
I just noticed that in your screen shot, the button that is normally "Reset" (lower-right corner) is instead "Set Default..." Normally, it only displays "Set Default..." when you hold down Alt (Opt on Mac). Were you holding down Alt when you took the screen shot?
I tried reproducing the problem by setting my display to be 1360x768 (Windows) and even smaller on Mac, but I couldn't trigger the bug.
You're very perceptive I was using ALT - Print Screen to take that screenshot. Still don't know the reason for it being the way it is, but at least I'm now able to see the scroll bar. Just in case anyone from Adobe is looking, here are some more screenshots to illustrate the problem.
Left panel - made it as narrow as it would go. No problems whatsoever.
Left panel - made it was wide as it would go. No problems whatsoever.
Right panel - made it as narrow as it would go. Vertical scroll bar disappears.
Right panel - made it as wide as it would go. Vertical scroll bar disappears. You can see the gap where the scroll bar should be.
For me to be able to see the vertical scroll bar on the right panel, I would either have to collapse the histogram which makes the scroll bar appear or resize the panel just right (not too narrow and not too wide).
This is a user-to-user forum in which Adobe product developers rarely participate. Perhaps you could post a bug report in the official Adobe feedback forum: Recently active topics in Photoshop Family about Photoshop Lightroom. Include the screen shots and the output from Help > System Info.
Okay, thanks for your help.