Would you mind rebooting and then reinstalling MC 16.0.39.
There are two links in my signature that might help. Stability and Weird Problems. Please try Stability first.
A couple of your problem suggest there might be something running in the background that is causing problems.
The video problem is probably a driver.
What virus checker are you using? Try uninstalling it just as a test.
In task manager is the CPU at 100% when you're having trouble?
Did you follow MS instructions on uninstalling IE9?
What you're seeing is far from normal, but if you can be patient, we can sort it out.
1. I've un/reinstalled three times, with complete reboots in between. No difference: the mouse-wheel scrolling effect still happens, and now I've noticed that Google Chrome becomes unresponsive in a variety of different ways--most notably, though, text-input boxes become completely unresponsive for long periods of time whenever MC16 is running. When MC16 is *not* running, everything is fine. The scrolling issue happens in all apps, though, not just Chrome. Is it possible that the Chromium rendering engine in MC16 is interacting weirdly with Chrome?
2. I checked out the Stability and Weird Issues articles, but didn't find anything in them that seemed related to the experiences I've been having. I went through the Stability article carefully and checked out everything I could there, but most of the points in the article were either ineffectual or unrelated.
3. The second time I reinstalled, the only apps running beside MC16 were AnyDVD HD 6.6.4.2, my Steam client, Microsoft Security Essentials, PowerISO, and AdAware. The behaviour still persisted. The third time, I also ran Google Chrome and Skype. In the latter case, the scroll problem was VERY noticeable in both apps. I imported a folder of about 1000 mp3 files just to see if the problem was caused during import, but it persisted looong after the import was done. In fact, I rebooted my computer a fourth time and checked MC16 after my boot sequence was completely finished: same thing.
4. I updated my video driver (GeForce 9600 GT) to the most recent stable release. No effect. I tried to play a simple .avi file and the weird jumping back and forth from display to desktop happened every couple of seconds.
5. I use Microsoft Security Essentials 64-bit, and I am LOATHE to turn it off for ANY software install. I disengaged it for a bit, though, and it didn't make any difference to MC16: the weird stuff still happened.
6. Oddly enough, no, it *doesn't* spike my processor usage--nor does it eat up an inordinate amount of memory. I've kept a close eye on my Task Manager's CPU graph and system resources during install and operation, and I haven't noticed any odd processor/memory performance issues.
7. Yep. To the letter. I didn't have any trouble with removing the update or running the restored version of IE8.
At present, I've got the software uninstalled and, honestly, have no interest in keeping it around. If you'd like, though, I'll reinstall it and try out any further suggestions you may have! If not, though, I'm going to wait until the next build and see if *that* makes any difference.
D.