Reveal cursor on panel expand

@peterflynn Simple fix for issue #3003, and (nearly) identical to closed and outdated pull request #3931.
I also wonder if trying to preserve the cursor's offset from the bottom edge is the right goal.  It might be better to just scroll the minimum amount needed to keep the cursor in view (so it would be on the bottommost line visible, just above the panel) -- this minimizes the amount that the editor jumps, and minimizes how far away from its original onscreen location the cursor winds up.  That actually seems better for the user (and might be simple to implement, too?).