There are a few thin lines below certain sentences. See the screenshot. Any idea what this is? It only shows when I select a part of the text.
The selection highlight will only highlight the area between the top and bottom of the font rendering height, meaning that any additional spacing that you add around that area is not highlighted. For example, setting your line-height multiplier to 1.5 would result in a half of the natural line height in between each line that won’t be drawn using the selection colour:
Smaller amounts of spacing may show up as little hairlines like this, and may not show up on every line on account of rounding variances—particularly when the text is zoomed in or out from its natural scale at an odd value, such as you depict, with 125% magnification.
Thanks, this is now clear.