[dredmorbius]

The rationalisation appears to be that there are two viewports, one left, one (when expanding sidebar items) right.

The left-hand scrollbar controls and reports on the left-hand viewport. The right-hand scrollbar the right-hand viewport.

More conventional organisation (and less confusing, possibly) would be to retain the scrollbars to the right of the corresponding viewport.

Note that left-handed scrollbars are occasionally used elsewhere, as in the X11 Athena Widgets (Xaw) set:

<https://en.wikipedia.org/wiki/X_Athena_Widgets>