Great idea, but it has to be optional. I personally would get pretty annoyed if the side panel opens every time i try to click on something near the side of the screen and accidentally overshoot it
I agree that this should be an optional setting in the Sidebar Settings. "Mouseover" > "Expand on mouseover" or "Do nothing on mouseover".
It would also be nice to see an additional configurable setting to set a millisecond delay to expand on mouseover. For example, if I want to mouseover the sidebar and quickly click another tab, it should not expand unless I hover my mouse in the sidebar for a configurable period of time (default 100ms?).