The taskbar has some powerful features that have become almost indispensable.
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Right click gives you a menu. In the past I've just used my mouse to roll up to the item I need and click to access. NOW if I roll my mouse up to the menu, as soon as I am off the taskbar icon the menu disappears (about 90% of the time).

I can access the menu (assuming I keep my cursor on the taskbar icon) with my up/down arrow keys but this is like stopping typing to write down a sticky note with a pen -- highly inefficient.

How can I get mouse access back?

I've not found anywhere that anyone has talked about this. I can find no control panel or registry values that would obviously affect this.

Note: If I create a new account on this same computer there is not a problem with mouse access so this is likely not a virus (and I do carry an up to date virus checker)