Thanks. Ubuntu’s default text editor did the job and presented it in a legible way.
That’s what my code looked like for one entry, but then the other one was a bit weird as if there was more going on that was possibly relevant, so that threw me. I’m at work now so can’t paste it unfortunately. I’ll have another dabble tonight.
You’d think what would work is temporarily disabling the visuals for that track, remove the HTML filter overlay via the menu then re-enabling video. Though somehow that still freezes Shotcut.