This is Extra Keys for Linux - based on the Windows edition of Extra Keys Pro but completely revised to run as a native Linux process.
Caution: this program will not work on every Desktop Environment. It uses XLib APIs and therefore needs Xorg display management (aka X11, X Server). Almost certainly will not work with Wayland! However, it has been tested on a number of popular Linux distros with various different Desktop Environments including Xfce, KDE, Fluxbox, Cinnamon, Gnome and Mate - all using Xorg of course.
It is completely safe to try out. If unwanted or not working, then it is easy to remove - just delete one or possibly two files and it's gone. The application consists only of an executable file and a settings file that is created on the first run. No changes are made to your system configuration.
Extra Keys Pro is the extended edition of the basic Extra Keys tool and the only edition for Linux. It has all the features of its Windows equivalent except for one difference, in that it does not have the option to type into a named window. It functions in the same way as a virtual keyboard and will enter characters into the currently active application window - exactly like the physical keyboard.
Extra Keys is not designed as an actual virtual keyboard, it simply gives quick access to characters that are not on the physical QWERTY keyboard. Although it has the option to output character codes in various formats, it is also not intended as an replacement for a full Character Map. It is just a simple alternative!
Installation is trivial, as it is portable software with only a single, self-contained executable file. Simply extract ExtraKeysProX from the download archive and save it to any convienient location. An alternative 32-bit executable is included in the archive if needed. There is also a template to make a desktop launcher icon, but only if wanted.
First time the program is run it creates a small settings file, extrakeyspro.cfg, which is the only other file used. At run time the program shows an icon in the notification area of the taskbar, enabling the keys panel to be hidden or shown on demand. This is usually no issue but some distros don't include the notification area by default, such as Trisquel with Mate, though very easy to add it.
All editions of Extra Keys are now free but if you find this software useful, please consider a donation to support further development
Just a small amount would help a lot.