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?).
I moved to Firefox from Microsoft Edge. Edge expands the sidebar on mouseover, which is awesome. I'm missing this feature in Firefox.
The rationale is: Sidebar has vertical tabs. Collapsed, you can't see website names, which you need to see when switching tabs. But after the tab switching is done, you don't need the sidebar expanded anymore, it just takes a lot of space. So, it makes sense to have an option that it autoexpands on hover and autocollapses on un-hover.