ZX81 video compatibility issues

[stevexyz]

For some reason the latest abuse of the standard video routine by drbeep (text 2 pixel horizontal scrolling) is not rendered properly on Clock Signal.

Below the video rendering of the BD.p program (cfr: https://www.sinclairzxworld.com/viewtopic.php?f=4&t=6179 where there is also how the video should be rendered):

[stevexyz]

Let's put the issue into pause, I'm trying to understand the behaviour of a real ZX81... (details in the discussion of the link provided)

[stevexyz]

Seems to work on real hardware (at least the 60hz version but the "trick" is the same on horizontal lines so should work also on 50)
Details in the discussion of the link provided