I made more tests on real ST + CRT for "No Buddies Land".
The game uses the hi/lo switch at pos 500, which creates a blank signal on next line and disrupt sync.
As we know, such sync disrupting are known to create horizontal bending of the display on the following lines.
In the case of the game, I think they were aware of this side effect, to minimize it on screen, they used several things :
- although only 5 lines max are required to move the screen vertically in that case, you can see that screen is black from lines 63 to ~83. This 20 black lines with no pixel shown will allow to hide the bending if it remains small
- scroll is not continuous : you only scroll when jumping at a certain point, then screen will scroll up quickly, so blank lines will be enabled for less than 1 sec, which will also lower the time during which hsync is altered, avoiding too much alteration of the screen (the game is made in such a way that it's very rare to be able to scroll up often).
On my CRT monitor, by turning luminosity to the max, I can the screen bending at line 63 when scrolling, but image remains stable (on the other end, the XXX international demo will fully bend my CRT when removing top border). The game also works fine on my LCD TV.
So, this method seems rather stable when compared to other lines where sync is altered. You just need a little extra cheating to hide the next lines and avoid showing too much of what is happening