Since the update to 12.04 I have had a couple seemingly related issues with my mouse.
1) When browsing a folder, it takes several times passing my mouse over a file before the file will 'light up' to indicate I'm pointing at it. Sometimes I can't get it to light up at all; sometimes I can get it to light up by pointing somewhere else. Unless it's lit up, clicking does not select it, and right-clicking brings up a context menu about the whole folder, not the individual file. I can still select files by dragging a selection box around them. Dragging files around has become impossible; instead, I get a selection box regardless of where I drag.
2) In some applications, such as Rhythmbox, merely passing the mouse over text (such as the title of the currently playing song) selects that text. It's a little bit like the mouse button is constantly held down; yet I have no abnormal behavior as far as selecting songs and playing them goes.
3) In GIMP, shortcuts like CTRL-S or CTRL-N don't work. Most mouse-based operations work fine, but when I try to crop, after selecting an area to crop, additional clicks resize the crop area more, rather than executing the crop like normal. This is strangely similar to everything being interpreted as a drag within folders, or else I wouldn't mention it.
4) The app menu, which I have set to auto-hide, does not appear when I move my mouse to the side or the corner of the screen.
5) In MyPaint, when the application starts up it believes the mouse button is being held down, and will not stop drawing no matter how I try clicking, until I move the mouse off the window and back on again. Less reproducibly, occasionally while drawing if I move the mouse off the window and wiggle it around, that gets interpreted as a brush stroke- one off where I can't see it, which I have to zoom out to find.
I think that's about it. Most applications behave fine; I don't notice any mouse anomalies when using Firefox or a PDF reader. I wouldn't hope for any suggestions on such an intermittent oddity if it weren't for the degree of commonality- the phenomenon of acting like I'm dragging seems to keep occurring.
All this behavior, by the way, is the same whether I'm using the touchpad, an external mouse, or a Wacom stylus.