My apologies. Perhaps my description is not clear. This behaviour has always been present for all versions and all file types. Anyway, I am on OS/X High Sierra (10.13.4). The version of slickedit is Slickedit Pro 2017 ( v22.0.2.1).
Briefly, I am vertically scrolling the edit window but the left display of the file wanders off under the left boundary of the window for a few characters, perhaps more. I attach a screen shot. NB I am using a Magic Trackpad 2 not a mouse.