But when they pan down and back up you can see there's no light show on at that time from the buildings. Also if a light show, the lights are colorful, and often strobing and rotating, but the triangle is static. I think lightshow is debunked.

But it's a fair guess. I also think that a static vertical spotlight atop a building could do this, but the air in Shanghai is not exactly particulate free and if you had these powerful light beams you'd expect to see a trace of them along the line from source to cloud, but there's no such. I think the ground-source idea is, well maybe not debunked but less likely than "odd triangular air mass", or CG. It could be a massive drone or something suspended from helicopters above it, maybe it could be like a light or firework platform that's being tested as part of the centenary celebrations?