Thanks everyone for the detailed answers! I just tested geany 1.38 (2022-05-01) from the official arch repo. The font there looks "sharp" as I wanted. It uses GTK+ v3.24.38, the same one I used building latest commit version from source. I tested geany with gtk2 too, font is sharp. I have to do some research and find out on which commit it starts blurry. It will take some time then I will write the results here.