I figured it out.
The text looks pixelated only when a sidebar is open on the left (e.g. the table of contents). When I close it and the text occupies the whole window, then it's readable and perfect. I didn't even try to close it, because in 1.7.2 I have it open all the time and got used to it.
So when I open the sidebar, it doesn't just reduce the textbox's size as it should, but tries to resize it instead, so the text gets pixelated and squashed. At least that's my theory.
I hope this bug gets fixed in the future.