Enabling Bias alt-tab sorting to prefer windows on the current viewport as described here made no difference. How can I get back the exact same behavior as 11.04, so that alt-tab only switches between windows on the current workspace?

Simply disabling the alt-tab and shift-alt-tab keybindings on the unity switcher seems to have helped, but it still switches workspaces on me sometimes. For example, if I give a terminal window focus then press alt-tab, it switches to another terminal window on any workspace before trying to switch on the same workspace. Also, the Unity switcher still shows up when I alt-tab then hold alt even though I removed its alt-tab keybinding.