Steps to reproduce:

1. Open attached project
2. Open 'Main' scene
3. Select 'Inspector Helper' in the hierarchy
4. Notice there are vertical, 1px wide *green* lines on the left and right of the Inspector drawn
(drawn between: 1 <= x <= 2 and (EditorGUIUtility.currentViewWidth - 4) <= x <= (EditorGUIUtility.currentViewWidth - 3))
5. Notice there are no vertical, 1px wide *red* lines on the left and right of the Inspector
(drawn between: 0 <= x <= 1 and (EditorGUIUtility.currentViewWidth - 1) <= x <= (EditorGUIUtility.currentViewWidth))

Open 'InspectorHelperEditor.cs' script to better inspect how are the mentioned lines drawn.

Note: The results are even worse if there is a scrollbar in the inspector (see attached gif).