Using Geany 2.0 (GTK+ v3.24.39 & GLib v2.78.3) on OpenSUSE Tumbleweed updated 1-19-2024, I see no graphical irregularities with Hack font size 9, 10, 12 or 14 using lines of underscores, Us, or random characters. System version of Pango is 1.51.1 and Harfbuzz is 8.3.0.