I’ve had a little more free time to investigate futher.
I’m using an Oppo Find X3 Pro running Android 13. I see the same effect at either Standard (2412x1080) or High (3216x1440) resolution. The browser window isn’t zoomed (100% web font size). I only see this in MS Edge. Chrome and Firefox both show the full, untruncated category names.
This is starting to sound like - Blue line above banner logo - support - Discourse Meta
That’s also only visible on Edge on the same phone. Edge was updated around the time I started to see the blue line, but I also upgraded my Discourse install to the latest patches at around the same time. Not sure what they might have done to make the browser behave so oddly?