Confirm with:
09:51:44: Geany INFO : Geany 1.37 (git >= 915cf6e8), en_AU.UTF-8 09:51:44: Geany INFO : GTK 3.24.20, GLib 2.64.3 09:51:44: Geany INFO : OS: Linux Mint 20 (ulyana)
a few line contents repeat, is fixed as soon as Scintilla is forced to re-compute the display, eg by a modification.
Will wait to see if #2600 fixes it.