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).