This issue applies to both Shell, with the prompt sidebar, and editor windows, with line number sidebars.  I looked carefully at Windows Notepad++ editor tabs.  They have a tab border, separate from the window border, that encompasses the line numbers, text, and scrollbar, but not the status bar.  The are no internal lines between the three components.  But the line numbers and scrollbar have a light gray background that separates them from the text.  

The IDLE text widgets instead have a border around the text but apparently not one around the larger frame.  As a result, text widgets with just a scrollbar do not look exactly right.  It would be easy to delete the text border and adjust the frame border. But as I suggested previously, I am pretty sure deleting it entirely would require a default light gray background.

Any change would have to be tested on all 3 major systems.