I added the following lines to my ~/.config/gtk-3.0/gtk.css:

TerminalScreenContainer

{ -TerminalScreenContainer-window-placement: bottom-right; -TerminalScreenContainer-window-placement-set: true; }

as a gnome web page says: https://wiki.gnome.org/Apps/Terminal/FAQ#How_can_I_move_the_scroll_bar_to_the_left_side_of_the_window_.28like_in_xterm.29.3F .
The above setting is supposed to move the scroll bar to the left side of the window. However, the scroll bar always appears on the right side.

–
I am reporting this issue here since gnome's bugzilla is only for gnome developers: https://wiki.gnome.org/Apps/Terminal/ReportingBugs .