Screen position accuracy problem in shader

So after isolating the code for the lensflare effect, I wasn’t able to reproduce the problem.
The effect appears to work correctly, so my code is apparently OK.
http://jsfiddle.net/vajhm7gt/47/
It must be related to something else in my code, but it’s pretty straightforward, the major difference is the use of ES Modules and a Class structure.
Closing for now.
Thanks for your replies.