I tried to re-create this lesson where you put in and out points in viewer, and then create clips and from them make your timeline. I discovered that at least on highly-compressed mkv file ("Swimming with orca in New Zealand-JQ3mDXF3bcE.mkv") setting position cursor (in viewer window) at random place with mouse and then trying to use frame forward/frame backward buttons doesn't work: I need to click many times until it finally (after hitting codec keyframe?) started to advance. I tried to set cache to 1 Mb - from 250 or 768Mb - bug was still around. Problem is - this is not just visual bug - rendered file also has this pause (I was surprized) Cinelerra Infinity - built: Sep 14 2020 05:20:48 - hopefully latest git. Anyone can confirm/deny?