Actually, it happens to me very often when I switch from one workspace to another using the keyboard shortcut Ctrl+Alt+Arrow. When I click, the menu shows up at first but immediately disappears once I roll the mouse over it.
Right now, to overcome it, I've been using Alt+Tab instead (which seems to work even when the target window is at a different workspace). With the workspace switcher from the unity launcher it seems to work fine as well.
Actually, it happens to me very often when I switch from one workspace to another using the keyboard shortcut Ctrl+Alt+Arrow. When I click, the menu shows up at first but immediately disappears once I roll the mouse over it.
Right now, to overcome it, I've been using Alt+Tab instead (which seems to work even when the target window is at a different workspace). With the workspace switcher from the unity launcher it seems to work fine as well.