In Ubuntu 12.04, my mouse clicks when I release the button.
What happens (bad): I right click (mousedown), the context menu appears, when I release the mouse button (mouseup), the item under the cursor is clicked.
What should happen (good): I right click (mousedown), the context menu appears, nothing happens when I release the button. To click an item in the context menu, I click it normally with the right mouse button.
I have experienced this behavior in Chrome, the file browser, and in gnome terminal. The mouse is a Razer DeathAdder (but I'm just running whatever drivers Ubuntu picked automatically), and if it matters, I'm using the AMD/ATI graphics drivers.
Best Answer
If you do not move your mouse in the meanwhile, there should not be any "menu item under cursor". The context menu opens such that its left upper corner is at the cursor, and there is a margin below to the next menu item.
However, if on mouse-button-release there is a menu item under the cursor, the desired behavior is to launch that menu item. This his how you (or at least many people) normally proceed: mouse-button-down, move the cursor to the desired menu item, mouse-button-release to activate that item.
If there is no menu-item under cursor after mouse-button-down, then the behavior is as you described: mouse-button-release does not destroy the context menu.
So maybe your mouse is very sensitive, and a button-down event is accompanied by a cursor move?