Increasing the font size does make the button visible but on my phone the blue line is visible at 100%. I need to drop the size to 90% to make it disappear, or set the font size in Discourse to Smaller. I’m of an age where I need to start increasing font size though so going smaller isn’t really an option.
I’m puzzled what the purpose of a hidden button is though. If most people (non-mobile Edge or with the ‘right’ font size) can’t see it then it doesn’t seem like a very useful UX item?