To help users understanding the correlation between the two columns in the diff view and the two pointers on the slider lines should show connecting both.
In our internal usability tests, the most frequent issue was that users were confused what the pointers actually influence. Lines are a a standard graphical device to show connections.
Details in the design depend on our impressions and minor tests to be done – for example, we could either show the lines all the time in the same color, or show them transparent most of the time and make them opaque when users hover or click the slider.
If the indicators are offscreen, I suggest the following: