Hello,
I first had the same thought, but actually that extra "bar" on the right side of the editor is quite a special feature:
this bar will indicate with
red areas any error locations in your file in case the "Error Analysis" (usually running
automatically - Tools|Settings|IDE|Code Insight|Instant error analysis" - but also available by menu Tools / Ctrl+Alt+V).
In case you see any of the red spots, move your mouse over it or click it to directly go to that location in your editor.
However, I couldn't find a way to drag it to the left border of the editor yet.
Cheers,
Tobias