When you move your mouse to the sides of a node the cursor changes into a "move" cursor and you can drag the edge left or right.
Unfortunately, that area is too wide and is only within the node contents area. The following image illustrates the current area on the left:
In a nutshell it starts in the middle of the border and extends into the content by 10 units. The biggest problem with this is that it will sometimes keep some content from working correctly if that area and the sizing area overlap. An example is the left side of the ColorRamp, which you cannot move.
The attached patch alters this so that the hit area is centered over the border, so works from the same distance on the left or right of the border. Specifically it makes it work from 3 units to the left to 3 units to the right, so still 6 units wide in total. The result is (roughly) illustrated by the right side of the attached image