Width: Surround alike effect that partly moves the phase of the signal to fool the brain. That’s also why its nearly not noticeable on PAs. You can create surround effects also from plain mono signals.
Stereo Expanders Expand: Simply boosts differences between the channels. Won’t do anything on mono signals.
Stereo Expanders Surround: Just like “Width”
The click comes from the fact that width is completely disabled at width 0 to avoid artifacts. So the width effect kicks in at “width > 0”.
Ideally we should have an on/off switch for the width, then a width amount slider to control it. On/Off can not be automated because it may click, while the amount can be automated. Not sure if thats really worth the troubles…