I have also uploaded a video of the behaviour when I set the timebase to .1ms and then play with the variable Time Div control - I can get the square wave to stabilise but its certainly not based on any trigger as it slowly moves. A Trig LEVEL makes no difference whatsoever. You will also notice that the trace fades towards the right hand side of the screen.
Have a look at
https://drive.google.com/file/d/1njT...ew?usp=sharing
I apologise for the background sounds - that is the sound of 40 pints of beer fermenting!