Maybe I've not noticed this before, but I pasted in a png with the top half invisible for masking purposes. When I look at this video in my timeline, I see a white horizontal line showing where my transparency in the png starts. This line does not output as part of the encoding, but it confuses me because I can't tell what really will be exported.
How can I toggle this line in my timeline viewer on and off. I did not see it in timeline options as something to change. Maybe I'm missing it.
In your linked clip you have resolution set to 'half' in your program monitor. Try 'Full' and see if the issue goes away. A slight adjustment of the size of your Program monitor window may also get rid of it.