Piotr, let's say pma_userconfig is undefined; you are still offering a Save button. You wrote in Documentation.html that settings will be saved in the browser's local storage or until the end of session but some users will be disappointed to see that these have not been saved permanently (some don't read the doc :) )
Maybe the "Save" button could be changed in this case to indicate the non-permanent status of this saving operation?
Also, when I save I get: ---------- Could not save configuration
#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '(`username`, `config_data`) VALUES ('root', 'a:1:{s:' at line 1 ---------- and ---------- Notice in ./libraries/user_preferences.lib.php#69 Undefined index: userconfig
Backtrace
./prefs_forms.php#58: PMA_save_userprefs(array) ----------