Comment 2 for bug 1755987

From https://askubuntu.com/questions/1153506/how-do-i-enable-all-keys-on-the-touchscreen-keyboard :

The GNOME On Screen Keyboard (OSK) is designed to generate keyboards from the Unicode Common Locale Data Repository data. The Unicode CLDR was never intended to limit the keys available, but that seems to be the path the GNOME developers are following. If a key isn't in the CLDR, there is no way to type it. So, that means, no Esc, no F1, no Control, etc.