How are you scrolling?

Separate Issue; Your System Details shows;

Installed Plug-ins

Adobe Shockwave for Director Netscape plug-in, version 12.2.1.171 Adobe Shockwave for Director Netscape plug-in, version 12.2.4.194
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.

Shockwave Director: Shockwave Standalone Installer / Uninstaller{web link} Shockwave Director Version Version 12.2.4.194 http://get.adobe.com/shockwave/

With the mouse wheel but it does the same thing if I use the down arrow key or if I click on the slider on the right side of the screen and drag it down.

I have discovered the problem, it lies with the YouTube+ add on. When I disabled the add on the problem stopped. I found a list of features relating to this add on and one is Fullbrowser mode. Below is the description.

Whenever the video starts playing the player will fill the entire space available in the browser. Once the video ends the player will return to its normal size. To return to the normal mode before the video ends simply press the Esc key or click the resize button present in the player, next to the Fullscreen button.

However, pressing the escape key did nothing nor did the resize button. It seems to be related to trying to run the add on while Flashplayer is being used. I found info that indicates all features of YT+ will only work if using HTML5.

Thank you to FredMcD who responded to my original query.

Glad to help. Safe Surfing.

Make sure that the player area doesn't have (get) focus as that may cause the player area to get zoomed if you rotate the wheel.