With a file full of lines of underscore there appears to be an extra wide gap between characters every so often which shows as a vertical line of background colour.
I think actually every so often the underscore is short because when U has the right upright in the same place it seems thinner.
Happens at the same place with `Hack regular` and `DejaVu sans mono book`. At 9 point it is every 4/5 characters at 10 point it is only column 19 and 57. With `Freemono regular` does not show at 10 point but does at 9 point at every 5 characters (not varying 4/5 like the other two fonts).
Happens with Geany 2.0 Scintilla 5.3.7. Does not happen with Vscode with same file and font and size. Does not happen with the old Scite on this system (4.3.0).
Maybe its a float rounding/truncation issue somewhere in Scintilla.
Before reporting it to Scintilla, is this visible to anyone else, and with Scite if you have the latest (I will try it when I get a chance to build it)?